common/rc: fix _xfs_is_realtime_file for internal rt devices

common/rc: fix _xfs_is_realtime_file for internal rt devices

Now that we can have internal realtime devices, it's possible to have a
realtime filesystem without setting USE_EXTERNAL=yes or SCRATCH_RTDEV.
Use the existing _xfs_has_feature helper to figure out if the given path
points to a filesystem with an online realtime volume.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>