exfat: fix the infinite loop in exfat_readdir()

In the Linux kernel, the following vulnerability has been resolved: 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.