xfs: don't propagate RTINHERIT -> REALTIME when there is no rtdev

While running generic/042 with -drtinherit=1 set in MKFS_OPTIONS, I
observed that the kernel will gladly set the realtime flag on any file
created on the loopback filesystem even though that filesystem doesn't
actually have a realtime device attached.  This leads to verifier
failures and doesn't make any sense, so be smarter about this.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>