exfat: add super block operations

This adds the implementation of superblock 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: 719c1e1829166da399903b13da6dcc5344e73449
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I290967b10b593b8836e0a4e2dd5f8035493952f7
Signed-off-by: lixiang <lixiang@codeaurora.org>