f2fs: notify when discard is not supported

Change f2fs so that a warning is emitted when an attempt is made to
mount a filesystem with the unsupported discard option.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>