fstests: require scratch dev in tests using dm targets

fstests: require scratch dev in tests using dm targets

We build dm device on top of scratch dev so we require $SCRATCH_DEV
to be a valid block device in _require_dm_target(). And we need to
_require_scratch before _require_dm_target, otherwise test fails if
there's no SCRATCH_DEV defined, where it should _notrun.

  +Usage: _require_block_device <dev>

So add _require_scratch_nocheck to generic/347 (we do the fs check
on thinp device), move _require_scratch before _require_dm_target in
xfs/006 and xfs/264.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>