ocfs2: fix boundary check in ocfs2_check_dir_entry() to use buffer offset

ocfs2: fix boundary check in ocfs2_check_dir_entry() to use buffer offset

Commit 390ac56cf0f6 ("ocfs2: add boundary check to
ocfs2_check_dir_entry()") added an out-of-bounds guard using the
caller-supplied 'offset' argument:

	if (offset > size - OCFS2_DIR_REC_LEN(1))
		return 0;

However, 'offset' and 'size' are not measured against the same base for
all callers.  In the block-based lookup path, ocfs2_find_entry_el() passes
'offset' as an absolute offset into the whole directory:

	i = ocfs2_search_dirblock(bh, dir, name, namelen,
				  block << sb->s_blocksize_bits,
				  bh->b_data, sb->s_blocksize, res_dir);

while 'size' is a single block size (sb->s_blocksize).  For any directory
entry located in the second or later block, 'offset' is >=
sb->s_blocksize, so the guard rejects every such entry even though it is
perfectly valid and lies entirely within its block buffer.

This makes mounting fail for filesystems whose system directory spans more
than one block, e.g.  a volume formatted with a small block size:

  mkfs.ocfs2 -b 512 -C 4096 -N 2 -T datafiles --fs-features=usrquota,grpquota

  ocfs2_check_dir_entry:314 ERROR: directory entry (#18: offset=512) too close to end or out-of-bounds
  ocfs2_init_local_system_inodes:496 ERROR: status=-22, sysfile=12, slot=0
  ocfs2_mount_volume:1757 ERROR: status = -22

The dirent's position within the buffer being validated is ((char *)de -
buf), which is what the rest of the function already uses (via
next_offset) and what must be bounds-checked against 'size'.  Compute that
buffer-relative offset and use it for the guard.  The subtraction is
reordered to size - buf_offset < OCFS2_DIR_REC_LEN(1) to avoid an unsigned
underflow when size is smaller than the minimal record length.

Link: https://lore.kernel.org/20260710040512.3310736-1-joseph.qi@linux.alibaba.com
Fixes: 390ac56cf0f6 ("ocfs2: add boundary check to ocfs2_check_dir_entry()")
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Dmitry Antipov <dmantipov@yandex.ru>
Tested-by: Dmitry Antipov <dmantipov@yandex.ru>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Heming Zhao <heming.zhao@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>