exfat: add exfat cache

This adds the implementation of exfat cache.

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: c35b6810c4952ae5776607e2c1d6a587425d5834
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I0c7fee26fbd277f4e2987c5bc11102b05194bd12
Signed-off-by: lixiang <lixiang@codeaurora.org>