fs/fat: remove unnecessary defintion

'*sb' never used, so let's remote it and pass inode->i_sb directly to the
MSDOS_SB.

Signed-off-by: Alexander Kuleshov <kuleshovmail@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>