xfs/424: don't use SCRATCH_DEV helpers

This tests forces external devices to be disabled by calling mkfs.xfs
directly and overriding SCRATCH_{LOG,RT}DEV, but the options specified in
MKFS_OPTIONS might not work for this configuration.  Instead hard code
the calls to xfs_db and don't modify the scratch device configuration.

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