fat: convert to unlocked_ioctl

FAT does not require the BKL in its ioctl function, which is already serialized
through a mutex. Since we're already touching the ioctl code, also fix the
missing handling of FAT_IOCTL_GET_ATTRIBUTES in the compat code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>