fs: fat: use MSDOS_SB macro to get msdos_sb_info

Use the MSDOS_SB macro to get msdos_sb_info, instead of coding it
directly.

Signed-off-by: Fred Chou <fred.chou.nd@gmail.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>