exfat: add in-memory and on-disk structures and headers

This adds in-memory and on-disk structures and headers.

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: 1acf1a564b6034b5af1e7fb23cb98cb3bb4f6003
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: Ibe7d599033d32965f9410cd58b5643704b0de46b
Signed-off-by: lixiang <lixiang@codeaurora.org>