fat: Add fat_msg() function for preformated FAT messages

Add fat_msg() to replace not cosequent used printk() in fs/fat/*
New message format should be as fallow:
FAT-fs (sda1): some thing happened.

Signed-off-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>