exfat: add directory operations

This adds the implementation of directory operations for exfat.

Signed-off-by: Namjae Jeon <[email protected]>
Signed-off-by: Sungjong Seo <[email protected]>
Reviewed-by: Pali Rohár <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Al Viro <[email protected]>

Author: namjaejeon