exfat: fix the infinite loop in exfat_readdir()

If the file system is corrupted so that a cluster is linked to
itself in the cluster chain, and there is an unused directory
entry in the cluster, 'dentry' will not be incremented, causing
condition 'dentry < max_dentries' unable to prevent an infinite
loop.

This infinite loop causes s_lock not to be released, and other
tasks will hang, such as exfat_sync_fs().

This commit stops traversing the cluster chain when there is unused
directory entry in the cluster to avoid this infinite loop.

Reported-by:  <syzbot+205c2644abdff9d3f9fc@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=205c2644abdff9d3f9fc


Tested-by:  <syzbot+205c2644abdff9d3f9fc@syzkaller.appspotmail.com>
Fixes: ca061973 ("exfat: add directory operations")
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>