exfat: add fat entry operations

This adds the implementation of fat entry 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: 31023864e67a5f390cefbe92f72343027dc3aa33
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I797356380a952b820e57195d4ba04e7ddc0cb7a7
Signed-off-by: lixiang <lixiang@codeaurora.org>