xfs/530: require a real SCRATCH_RTDEV

xfs/530: require a real SCRATCH_RTDEV

Require a real SCRATCH_RTDEV instead of faking one up using a loop
device, as otherwise the options specified in MKFS_OPTIONS might
not actually work the configuration.

Note that specifying a rtextsize doesn't work for zoned file systems,
so _notrun when mkfs fails.

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