xfs/528: 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. Signed-off-by: Christoph Hellwig <[email protected]> Reviewed-by: Darrick J. Wong <[email protected]> Signed-off-by: Zorro Lang <[email protected]>