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.

fsck/main.c

@@ -4,6 +4,7 @@
  */
 
 #include "fsck.h"
+#include "misc.h"
 #include <getopt.h>
 #include <sys/resource.h>
 #include <sys/mman.h>
@@ -198,7 +199,7 @@ static char *parse_options(struct fsck_data *data, int argc, char *argv[])
 			break;
 #endif
 		case 'd':	/* --create-passes-dump */
-			asprintf(&data->rebuild.passes_dump_file_name, "%s",
+			xasprintf(&data->rebuild.passes_dump_file_name, "%s",
 				 optarg);
 			data->options |= OPT_SAVE_PASSES_DUMP;
 			break;
@@ -208,7 +209,7 @@ static char *parse_options(struct fsck_data *data, int argc, char *argv[])
 			break;
 
 		case 'h':	/* --hash: suppose that this hash was used on a filesystem */
-			asprintf(&data->rebuild.defined_hash, "%s", optarg);
+			xasprintf(&data->rebuild.defined_hash, "%s", optarg);
 			if (name2func(data->rebuild.defined_hash) == NULL)
 				reiserfs_panic
 				    ("reiserfsck: Unknown hash is specified: %s",
@@ -221,12 +222,12 @@ static char *parse_options(struct fsck_data *data, int argc, char *argv[])
 			break;
 
 		case 'R':	/* preparing rollback data */
-			asprintf(&data->rebuild.rollback_file, "%s", optarg);
+			xasprintf(&data->rebuild.rollback_file, "%s", optarg);
 			data->options |= OPT_SAVE_ROLLBACK;
 			break;
 
 		case 'B':	/* list of phisically corrupted blocks */
-			asprintf(&badblocks_file, "%s", optarg);
+			xasprintf(&badblocks_file, "%s", optarg);
 			data->options |= BADBLOCKS_FILE;
 			break;
 
@@ -916,25 +917,29 @@ static void rebuild_tree(reiserfs_filsys_t fs)
 		      "reiserfsck --rebuild-tree started at %s"
 		      "###########\n", ctime(&t));
 
+	/* Deliberate cascade: each pass falls into the next so that a rebuild
+	   always runs every stage from the requested starting point on. */
 	switch (where_to_start_from(fs)) {
 	case START_FROM_THE_BEGINNING:
 		reset_super_block(fs);
 		pass_0(fs);
-
+		/* fall through */
 	case START_FROM_PASS_1:
 		reset_super_block(fs);
 		pass_1(fs);
-
+		/* fall through */
 	case START_FROM_PASS_2:
 		pass_2(fs);
-
+		/* fall through */
 	case START_FROM_SEMANTIC:
 		pass_3_semantic(fs);
+		/* fall through */
 
 		/* if --lost+found is set - link unaccessed directories to lost+found
 		   directory */
 	case START_FROM_LOST_FOUND:
 		pass_3a_look_for_lost(fs);
+		/* fall through */
 
 	case START_FROM_PASS_4:
 		/* 4. look for unaccessed items in the leaves */
@@ -1267,7 +1272,7 @@ static int open_devices_for_rollback(char *file_name, struct fsck_data *data)
 	fs = getmem(sizeof(*fs));
 	fs->fs_dev = fd;
 	fs->fs_vp = data;
-	asprintf(&fs->fs_file_name, "%s", file_name);
+	xasprintf(&fs->fs_file_name, "%s", file_name);
 
 	if (data->journal_dev_name && strcmp(data->journal_dev_name, file_name)) {
 		fs->fs_journal_dev = open(data->journal_dev_name,
@@ -1454,7 +1459,8 @@ int main(int argc, char *argv[])
 		if (auto_check(fs))
 			break;
 		data->mode = FSCK_FIX_FIXABLE;
-	case FSCK_CHECK:
+		/* fall through */
+	case FSCK_CHECK:  /* and FSCK_FIX_FIXABLE */
 	case FSCK_FIX_FIXABLE:
 		check_fs(fs);
 		break;