Recent testers were slightly confused that a realtime mount failed
due to missing CONFIG_XFS_RT; we can make that a little more
obvious.

V2: drop the else as suggested by Christoph
Signed-off-by: NEric Sandeen <sandeen@sandeen.net>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAlex Elder <aelder@sgi.com>