fat: notify when discard is not supported

Change fatfs so that a warning is emitted when an attempt is made to mount
a filesystem with the unsupported `discard' option.

ext4 aready does this: http://patchwork.ozlabs.org/patch/192668/



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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>