ocfs2: validate cl_bpc in allocator inodes to prevent divide-by-zero

The chain allocator field cl_bpc (blocks per cluster) is read from disk
and used in division operations without validation. A corrupted
filesystem image with cl_bpc=0 causes a divide-by-zero crash in the
kernel:

divide error: 0000 [#1] PREEMPT SMP KASAN
RIP: 0010:ocfs2_bg_discontig_add_extent fs/ocfs2/suballoc.c:335 [inline]
RIP: 0010:ocfs2_block_group_fill+0x5bd/0xa70 fs/ocfs2/suballoc.c:386
Call Trace:
ocfs2_block_group_alloc+0x7e9/0x1330 fs/ocfs2/suballoc.c:703
ocfs2_reserve_suballoc_bits+0x20a6/0x4640 fs/ocfs2/suballoc.c:834
ocfs2_reserve_new_inode+0x4f4/0xcc0 fs/ocfs2/suballoc.c:1074
ocfs2_mknod+0x83c/0x2050 fs/ocfs2/namei.c:306

This patch adds validation in ocfs2_validate_inode_block() to ensure
cl_bpc matches the expected value calculated from the superblock's cluster
size and block size for chain allocator inodes (identified by
OCFS2_CHAIN_FL).

Moving the validation to inode validation time (rather than allocation time)
has several benefits:
- Validates once when the inode is read, rather than on every allocation
- Protects all code paths that use cl_bpc (allocation, resize, etc.)
- Follows the existing pattern of inode validation in OCFS2
- Centralizes validation logic

The validation catches both:
- Zero values that cause divide-by-zero crashes
- Non-zero but incorrect values indicating filesystem corruption or
mismatched filesystem geometry

With this fix, mounting a corrupted filesystem produces:

OCFS2: ERROR (device loop0): ocfs2_validate_inode_block: Inode 74
has corrupted cl_bpc: ondisk=0 expected=16

instead of a kernel crash.

[dmantipov@yandex.ru: combine into the series and tweak the message to fit the commonly used style]
Link: https://lkml.kernel.org/r/20251030153003.1934585-2-dmantipov@yandex.ru
Link: https://lore.kernel.org/ocfs2-devel/20251026132625.12348-1-kartikey406@gmail.com/T/#u [v1]
Link: https://lore.kernel.org/all/20251027124131.10002-1-kartikey406@gmail.com/T/ [v2]
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reported-by: syzbot+fd8af97c7227fe605d95@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fd8af97c7227fe605d95
Tested-by: syzbot+fd8af97c7227fe605d95@syzkaller.appspotmail.com
Suggested-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Heming Zhao <heming.zhao@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Mark Fasheh <mark@fasheh.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>