Support clang builds; fix the defects clang and its sanitizers exposed

Clang (22.1.8 on the host, 14.0.6 in the Debian VM) now builds the tree
warning-free with --enable-warnings, in-tree and out-of-tree, with -j, and
the full VM battery passes with CC=clang: 12/12. ASan/UBSan builds run the
real tool paths (mkfs, check, tune, resize, rollback save/restore/reject)
with halt_on_error and report zero errors. Documented in tests/vm/RESULTS.md;
the CI gained clang+warnings, out-of-tree and sanitizer jobs.

Fixes found by this work:

- reiserfscore/Makefile.am: reiserfs_err.c and reiserfs_err.h shared one
  recipe. With -j, make runs the recipe once per stale target, so two
  compile_et invocations race: one deletes reiserfs_err.c ("no input
  files") while another job compiles it. Attached the recipe to
  reiserfs_err.c only; reiserfs_err.h merely depends on it. Reproduced
  with clang+ASan, stable across 5 clang / 3 asan / 3 gcc parallel builds.

- reiserfscore/Makefile.am: the rule read the source-local
  reiserfs_err.et, so VPATH builds could not find it. Use $(srcdir)/.
  make distcheck now passes.

- reiserfscore/reiserfslib.c: reiserfs_close() did fsync(fs->fs_dev) after
  reiserfs_free() released fs -- a 4-byte heap-use-after-free, flagged
  exactly at reiserfslib.c:406/407 by ASan (found by clang+ASan; gcc never
  caught it because the freed heap usually still holds the old value).
  Moved fsync before the free; reiserfs_flush() has already written
  journal and dirty buffers at that point and free_buffers() never
  writes, so the ordering preserves durability semantics.

- fsck/semantic_check.c, fsck/semantic_rebuild.c: three %K arguments passed
  &tmp_deh->deh2_dir_id where a struct reiserfs_key * is expected. It
  printed the right values only because deh2_dir_id sits at offset 4 and
  the key is read from the pointer as if dir_id sat at offset 0. Build a
  proper local key with set_key_dirid()/set_key_objectid() (the same
  idiom debugreiserfs/scan.c already uses). Verified -Waddress-of-packed-
  member (clang-only) is gone and output is unchanged.

- reiserfscore/hashes.c: four unreachable *(int *)0 = 0 stores (guarded by
  len >= 16/12/8/4 tests that cannot hold after the while (len >= 16)
  loop, len <= 15). Removed; clang's -Wnull-dereference goes quiet.
  Behavior verified: keyed_hash/yura_hash/r5_hash outputs for 16 inputs
  from 0 to 17 bytes are byte-identical before and after.

run_in_vm.sh: propagate CC into the guest, force a remount of /mnt/data
(the host rewrites the backing file under the guest), exclude generated
reiserfs_err.[ch] from the source tar. run_tests.sh: report which compiler
the battery actually used.

fsck/semantic_rebuild.c

@@ -1006,11 +1006,23 @@ start_again:			/* when directory was relocated */
 				} else {
 					/* update key dir entry points to */
 					struct reiserfs_de_head *tmp_deh;
+					struct reiserfs_key deh_key;
 
 					tmp_deh =
 					    B_I_DEH(get_bh(&path),
 						    tp_item_head(&path)) +
 					    path.pos_in_item;
+					/* Build a proper key from the entry's
+					   dirid/objectid.  Passing
+					   &tmp_deh->deh2_dir_id as a
+					   reiserfs_key * only worked by
+					   offset coincidence and trips
+					   -Waddress-of-packed-member. */
+					set_key_dirid(&deh_key,
+						      get_deh_dirid(tmp_deh));
+					set_key_objectid(&deh_key,
+							 get_deh_objectid(tmp_deh));
+
 					fsck_log
 					    ("The entry %K (\"%.*s\") in directory %K updated to point to ",
 					     &object_key, namelen, name,
@@ -1023,7 +1035,7 @@ start_again:			/* when directory was relocated */
 							 (&relocated_ih.
 							  ih_key));
 
-					fsck_log("%K\n", &tmp_deh->deh2_dir_id);
+					fsck_log("%K\n", &deh_key);
 					mark_buffer_dirty(get_bh(&path));
 				}
 				dir_size += DEH_SIZE + entry_len;