xfs/424: disable external devices

xfs/424: disable external devices

This test uses an open-coded call to mkfs, so we need to disable the
external devices so that _scratch_xfs_db doesn't get confused.  We also
disable the post-check fsck because it's run by the parent ./check
program, which won't know that we didn't use the external devices.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>