exfat: add directory operations

This adds the implementation of directory operations for exfat.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Git-commit: ca06197382bde0a3bc20215595d1c9ce20c6e341
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I7dfcff9934c3cf0a048a8b754f9f934f3b441dfe
Signed-off-by: lixiang <lixiang@codeaurora.org>