exfat: add file operations

This adds the implementation of file 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>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Git-commit: 98d917047e8b7f4bb2ff61d81b0ccd94a00444f9
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I11d7ba143f67f698995b7a91e52ce26272ac9ff4
Signed-off-by: lixiang <lixiang@codeaurora.org>