exfat: fix reporting fs error when reading dir beyond EOF

Since seekdir() does not check whether the position is valid, the
position may exceed the size of the directory. We found that for
a directory with discontinuous clusters, if the position exceeds
the size of the directory and the excess size is greater than or
equal to the cluster size, exfat_readdir() will return -EIO,
causing a file system error and making the file system unavailable.

Reproduce this bug by:

seekdir(dir, dir_size + cluster_size);
dirent = readdir(dir);

The following log will be printed if mount with 'errors=remount-ro'.

[11166.712896] exFAT-fs (sdb1): error, invalid access to FAT (entry 0xffffffff)
[11166.712905] exFAT-fs (sdb1): Filesystem has been set read-only

Fixes: 1e5654de0f51 ("exfat: handle wrong stream entry size in exfat_readdir()")
Cc: [e-mail] # v5.7+
Signed-off-by: Yuezhang Mo <[e-mail]>
Reviewed-by: Andy Wu <[e-mail]>
Reviewed-by: Aoyama Wataru <[e-mail]>
Reviewed-by: Sungjong Seo <[e-mail]>
Signed-off-by: Namjae Jeon <[e-mail]>

fs/exfat/dir.c

···
 			clu.dir = ei->hint_bmap.clu;
 		}
 
-		while (clu_offset > 0) {
+		while (clu_offset > 0 && clu.dir != EXFAT_EOF_CLUSTER) {
 			if (exfat_get_next_cluster(sb, &(clu.dir)))
 				return -EIO;