exfat: fix the infinite loop in exfat_find_last_cluster()

In exfat_find_last_cluster(), the cluster chain is traversed until
the EOF cluster. If the cluster chain includes a loop due to file
system corruption, the EOF cluster cannot be traversed, resulting
in an infinite loop.

If the number of clusters indicated by the file size is inconsistent
with the cluster chain length, exfat_find_last_cluster() will return
an error, so if this inconsistency is found, the traversal can be
aborted without traversing to the EOF cluster.

Reported-by:  <syzbot+f7d147e6db52b1e09dba@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=f7d147e6db52b1e09dba


Tested-by:  <syzbot+f7d147e6db52b1e09dba@syzkaller.appspotmail.com>
Fixes: 31023864 ("exfat: add fat entry operations")
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>