generic: _require_dm_target() helper

generic/085 was failing on a machine w/o devicemapper kernel
support because it requires the linear target, but didn't
explicitly test for it.

I could have cut & pasted _require_dm_linear(), but chose
to go the route of a generic helper, _require_dm_target $FOO,
because some day someone will need the zero target, the error
target, or who knows.

Add the helper, use it in test generic/085, and convert
_require_dm_flakey, _require_dm_snapshot, and
_dmerror_required with this new helper.

Reported-by: Angelo Dureghello <angelo.dureghello@nomovok.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/generic/085

@@ -41,6 +41,16 @@ _cleanup()
 	cleanup_dmdev
 }
 
+cleanup_dmdev()
+{
+	# in case it's still suspended and/or mounted
+	$DMSETUP_PROG resume $lvdev >/dev/null 2>&1
+	$UMOUNT_PROG $lvdev >/dev/null 2>&1
+
+	$DMSETUP_PROG remove $node >>$seqres.full 2>&1
+	$DMSETUP_PROG mknodes >/dev/null 2>&1
+}
+
 # get standard environment, filters and checks
 . ./common/rc
 . ./common/filter
@@ -50,7 +60,7 @@ _supported_fs generic
 _supported_os Linux
 _require_scratch
 _require_block_device $SCRATCH_DEV
-_require_command $DMSETUP_PROG
+_require_dm_target linear
 _require_freeze
 
 setup_dmdev()
@@ -61,16 +71,6 @@ setup_dmdev()
 	return $?
 }
 
-cleanup_dmdev()
-{
-	# in case it's still suspended and/or mounted
-	$DMSETUP_PROG resume $lvdev >/dev/null 2>&1
-	$UMOUNT_PROG $lvdev >/dev/null 2>&1
-
-	$DMSETUP_PROG remove $node >>$seqres.full 2>&1
-	$DMSETUP_PROG mknodes >/dev/null 2>&1
-}
-
 rm -f $seqres.full
 echo "Silence is golden"