fat: simplify writeback_inode function

Simplify writeback_inode function.

Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Amit Sahrawat <amit.sahrawat83@gmail.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>