ocfs2: fix kernel BUG in ocfs2_find_victim_chain

syzbot reported a kernel BUG in ocfs2_find_victim_chain() because the
`cl_next_free_rec` field of the allocation chain list (next free slot in
the chain list) is 0, triggring the BUG_ON(!cl->cl_next_free_rec)
condition in ocfs2_find_victim_chain() and panicking the kernel.

To fix this, an if condition is introduced in ocfs2_claim_suballoc_bits(),
just before calling ocfs2_find_victim_chain(), the code block in it being
executed when either of the following conditions is true:

1. `cl_next_free_rec` is equal to 0, indicating that there are no free
chains in the allocation chain list
2. `cl_next_free_rec` is greater than `cl_count` (the total number of
chains in the allocation chain list)

Either of them being true is indicative of the fact that there are no
chains left for usage.

This is addressed using ocfs2_error(), which prints
the error log for debugging purposes, rather than panicking the kernel.

Link: https://lkml.kernel.org/r/20251201130711.143900-1-activprithvi@gmail.com
Signed-off-by: Prithvi Tambewagh <activprithvi@gmail.com>
Reported-by: syzbot+96d38c6e1655c1420a72@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=96d38c6e1655c1420a72
Tested-by: syzbot+96d38c6e1655c1420a72@syzkaller.appspotmail.com
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
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>

Author: rkt-1597

fs/ocfs2/suballoc.c

@@ -1993,6 +1993,16 @@ static int ocfs2_claim_suballoc_bits(struct ocfs2_alloc_context *ac,
 	}
 
 	cl = (struct ocfs2_chain_list *) &fe->id2.i_chain;
+	if (!le16_to_cpu(cl->cl_next_free_rec) ||
+	    le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)) {
+		status = ocfs2_error(ac->ac_inode->i_sb,
+				     "Chain allocator dinode %llu has invalid next "
+				     "free chain record %u, but only %u total\n",
+				     (unsigned long long)le64_to_cpu(fe->i_blkno),
+				     le16_to_cpu(cl->cl_next_free_rec),
+				     le16_to_cpu(cl->cl_count));
+		goto bail;
+	}
 
 	victim = ocfs2_find_victim_chain(cl);
 	ac->ac_chain = victim;