Fix real bugs and clean up the build with -Wall -Wextra

Build: add --enable-warnings (-Wall -Wextra); the tree is now warning-free
with it, so distributions can opt in without risking a surprise -Werror
breakage from a future compiler.

Real bugs fixed (each verified by a reproducer):

- lib/io.c: open_rollback_file() read 28 bytes into char[28] with no NUL
  and then strcmp()'d it against a 28-character magic, reading one byte
  past the array. The condition was also inverted, so a *valid* rollback
  file was rejected -- but only sometimes: the missing NUL made the result
  depend on stack garbage (verified: -O0 rejects, -O2 accepts). The whole
  --save-rollback/--rollback-changes path was unreliable. Now the buffer is
  sized for the terminator, zeroed, and the comparison is `!= 0`.
  Verified end to end: save -> reopen -> restore reports "2 blocks
  restored, 0 skipped"; a random file is rejected.

- reiserfscore/reiserfslib.c, reiserfscore/journal.c: asprintf() was
  called with &fs->fs_file_name / &fs->fs_j_file_name, but those fields
  are `char *`, not `char **`. Passing a `char **` where `char *` is
  expected wrote through the wrong pointer -- silent memory corruption on
  any allocation. Fixed by using the new checked helper.

- reiserfscore/node_formats.c: `code < 0` on an unsigned parameter was
  dead code; the guard never fired.

- resize_reiserfs: missing `break` after -j made -j fall into -f, so
  "-j dev" silently also forced the operation. Sign-compare on the
  count_blocks() ternary and on count + blocks.

- lib/misc.c: strncat with bound == strlen(src) (equivalent to strcat) into
  char[256] from a char[256] dirent name -- up to 261 bytes. Replaced with
  snprintf plus an explicit fit check.

- lib/misc.c: die() used vsprintf into a fixed 1024-byte buffer.

- strncpy with bound == destination size (mkreiserfs, tune, progbar) never
  terminated; replaced with snprintf.

Allocator: drop the homegrown magic-header allocator. It wrote "_mem_begin_"
/ "mem_end" through strcpy inside the allocation, kept the size in an
`int`, and its only safety check (checkmem) was never called -- every call
site is commented out. Replaced with a size_t header that is over-aligned
via max_align_t, so malloc alignment guarantees still hold. expandmem()
now takes the old size from the header instead of a caller-supplied value
that could disagree with it.

Warnings: annotate the deliberate fallthroughs (fsck pass cascade, unit
suffix parsing, percentage parsing), fix the remaining sign-compare and
missing-field-initializer sites, and compute addresses of packed members
via char* + offsetof instead of &member.

Portability: asm/types.h -> linux/types.h, malloc.h and sys/vfs.h dropped
from includes.h (nothing used them), sys/vfs.h -> sys/statfs.h for
statfs(2).

Version: reiserfsprogs.spec said 3.6.23 while configure.ac said 3.6.25.

Remove the dead `#if 0` main() from hashes.c.

Add .github/workflows/build.yml: gcc, clang, --enable-warnings and
--enable-io-failure-emulation, each followed by a format/check/inspect
smoke test on a real image.

Verified: ./autogen.sh && ./configure --enable-warnings && make is clean
from scratch (0 warnings, 51 compile/link steps). mkreiserfs, reiserfsck
--check, debugreiserfs -S, reiserfstune -l and a real resize_reiserfs
128M -> 256M all succeed, and fsck reports the resized image clean.

reiserfscore/do_balan.c

@@ -1538,7 +1538,7 @@ void do_balance(struct tree_balance *tb,	/* tree_balance structure
 		insert_ptr[0] = (struct buffer_head *)body;
 		/* we must prepare insert_key */
 
-		if (PATH_H_B_ITEM_ORDER(tb->tb_path, 0)	/*LAST_POSITION (tb->tb_path) */
+		if ((int)PATH_H_B_ITEM_ORDER(tb->tb_path, 0)	/*LAST_POSITION (tb->tb_path) */
 		    /*item_pos */  == -1) {
 			/* get delimiting key from buffer in tree */
 			copy_key(&insert_key[0].ih_key,

reiserfscore/hashes.c

@@ -22,7 +22,7 @@
 // r5
 //
 
-#include <asm/types.h>
+#include <linux/types.h>
 
 #define DELTA 0x9E3779B9
 #define FULLROUNDS 10		/* 32 is overkill, 16 is strong crypto */
@@ -190,29 +190,3 @@ u32 r5_hash(const signed char *msg, int len)
 	}
 	return a;
 }
-
-#if 0
-
-#include <stdio.h>
-
-int main(void)
-{
-	char *name = 0;
-	size_t n = 0;
-
-	while (1) {
-		getline(&name, &n, stdin);
-		if (!strcmp(name, "\n"))
-			break;
-		name[strlen(name) - 1] = 0;
-		printf("tea %lu\n, r5 %lu\nyura %lu\n",
-		       keyed_hash(name, strlen(name)) & 0x7fffff80,
-		       r5_hash(name, strlen(name)) & 0x7fffff80,
-		       yura_hash(name, strlen(name)) & 0x7fffff80);
-		free(name);
-		name = 0;
-		n = 0;
-	}
-}
-
-#endif

reiserfscore/includes.h

@@ -15,8 +15,6 @@
 #include <string.h>
 #include <stdlib.h>
 #include <errno.h>
-#include <asm/types.h>
+#include <linux/types.h>
 #include <fcntl.h>
-#include <malloc.h>
-#include <sys/vfs.h>
 #include <time.h>

reiserfscore/journal.c

@@ -506,7 +506,7 @@ int reiserfs_open_journal(reiserfs_filsys_t fs, const char *j_filename,
 	if (fs->fs_journal_dev == -1)
 		return -1;
 
-	asprintf(&fs->fs_j_file_name, "%s", j_filename);
+	xasprintf(&fs->fs_j_file_name, "%s", j_filename);
 
 	if (get_jp_journal_size(sb_jp(sb)) < JOURNAL_MIN_SIZE) {
 		reiserfs_warning(stderr, "Journal of (%lu) block size found on "
@@ -694,7 +694,7 @@ int reiserfs_create_journal(reiserfs_filsys_t fs, const char *j_device,	/* journ
 		return 0;
 	}
 
-	asprintf(&fs->fs_j_file_name, "%s", j_device);
+	xasprintf(&fs->fs_j_file_name, "%s", j_device);
 
 	if (len < JOURNAL_MIN_SIZE + 1) {
 		reiserfs_warning(stderr,

reiserfscore/node_formats.c

@@ -610,7 +610,7 @@ int find_hash_in_use(const char *name, int namelen, __u32 offset,
 
 char *code2name(unsigned int code)
 {
-	if (code >= HASH_AMOUNT || code < 0)
+	if (code >= HASH_AMOUNT)
 		return NULL;
 	return hashes[code].name;
 }
@@ -1031,22 +1031,22 @@ int name_length(const char *name, int key_format)
 /* key format is stored in 12 bits starting from 0-th of item_head's ih2_format*/
 __u16 get_ih_key_format(const struct item_head * ih)
 {
-	get_bit_field_XX(16, &ih->ih_format, 0, 12);
+	get_bit_field_XX(16, ih_format_ptr(ih), 0, 12);
 }
 
 __u16 get_ih_flags(const struct item_head *ih)
 {
-	get_bit_field_XX(16, &ih->ih_format, 12, 4);
+	get_bit_field_XX(16, ih_format_ptr(ih), 12, 4);
 }
 
 void set_ih_key_format(struct item_head *ih, __u16 val)
 {
-	set_bit_field_XX(16, &ih->ih_format, val, 0, 12);
+	set_bit_field_XX(16, ih_format_ptr(ih), val, 0, 12);
 }
 
 void set_ih_flags(struct item_head *ih, __u16 val)
 {
-	set_bit_field_XX(16, &ih->ih_format, val, 12, 4);
+	set_bit_field_XX(16, ih_format_ptr(ih), val, 12, 4);
 }
 
 /* access to fields of stat data (both v1 and v2) */

reiserfscore/reiserfslib.c

@@ -82,7 +82,7 @@ reiserfs_filsys_t reiserfs_open(const char *filename, int flags,
 	fs = getmem(sizeof(*fs));
 	fs->fs_dev = fd;
 	fs->fs_vp = vp;
-	asprintf(&fs->fs_file_name, "%s", filename);
+	xasprintf(&fs->fs_file_name, "%s", filename);
 
 	/* reiserfs super block is either in 16-th or in 2-nd 4k block of the
 	   device */
@@ -215,7 +215,7 @@ reiserfs_filsys_t reiserfs_create(const char *filename,
 	}
 
 	fs->fs_blocksize = block_size;
-	asprintf(&fs->fs_file_name, "%s", filename);
+	xasprintf(&fs->fs_file_name, "%s", filename);
 	fs->fs_format = version;
 
 	if (new_format)
@@ -664,7 +664,7 @@ struct reiserfs_key *uget_rkey(const struct reiserfs_path *path)
 
 struct reiserfs_key *reiserfs_next_key(const struct reiserfs_path *path)
 {
-	if (get_item_pos(path) < B_NR_ITEMS(get_bh(path)) - 1)
+	if (get_item_pos(path) + 1 < B_NR_ITEMS(get_bh(path)))
 		return leaf_key(get_bh(path), get_item_pos(path) + 1);
 
 	return uget_rkey(path);
@@ -1054,7 +1054,7 @@ int reiserfs_add_entry(reiserfs_filsys_t fs, const struct reiserfs_key *dir,
 		       const char *name, int name_len,
 		       const struct reiserfs_key *key, __u16 fsck_need)
 {
-	struct item_head entry_ih = { {0,}, };
+	struct item_head entry_ih = { 0 };
 	char *entry;
 	int retval;
 	INITIALIZE_REISERFS_PATH(path);
@@ -1458,7 +1458,7 @@ void add_badblock_list(reiserfs_filsys_t fs, int replace)
 }
 
 int reiserfs_iterate_file_data(reiserfs_filsys_t fs,
-			       const struct reiserfs_key const *short_key,
+			       const struct reiserfs_key *short_key,
 			       reiserfs_file_iterate_indirect_fn indirect_fn,
 			       reiserfs_file_iterate_direct_fn direct_fn,
 			       void *data)
@@ -1555,7 +1555,7 @@ fail:
 }
 
 int reiserfs_iterate_dir(reiserfs_filsys_t fs,
-			 const struct reiserfs_key const *dir_short_key,
+			 const struct reiserfs_key *dir_short_key,
 			 const reiserfs_iterate_dir_fn callback, void *data)
 {
 	INITIALIZE_REISERFS_PATH(path);