xfs/065: make sure SCRATCH_DEV is unmount before mkfs

xfs/065: make sure SCRATCH_DEV is unmount before mkfs

Commit "8309b39a fstests: fix broken _require_scratch usage" did
below change on xfs/065:

  -_scratch_unmount 2>/dev/null
  +_scratch_mkfs_xfs >> $seqres.full

It cause xfs/065 always fails now, as:
  QA output created by 065
  mkfs.xfs: /dev/sdb2 contains a mounted filesystem
  ...

So use _require_scratch, to make sure the SCRATCH_DEV is unmounted
before mkfs.

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