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>

fs/fat/inode.c

@@ -580,7 +580,7 @@ static void fat_set_state(struct super_block *sb,
 {
 	struct buffer_head *bh;
 	struct fat_boot_sector *b;
-	struct msdos_sb_info *sbi = sb->s_fs_info;
+	struct msdos_sb_info *sbi = MSDOS_SB(sb);
 
 	/* do not change any thing if mounted read only */
 	if ((sb->s_flags & MS_RDONLY) && !force)