exfat: add directory operations

This adds the implementation of directory operations for exfat.
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: NSungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: NPali Rohár <pali.rohar@gmail.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>