common: use mount/umount helpers everywhere

Replace every explicit mount/umount of scratch or test devices with
helper functions. This allows the next patch to add in hooks to these
functions in order to set up & tear down overlayfs on every mount/umount

(also adds _test_unmount(), which didn't exist prior)

[Eryu Guan rebased the patch agains latest master and replaced more
mount/umount with helpers]

Signed-off-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eric Sandeen <esandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

common/filestreams

@@ -225,5 +225,5 @@ _cleanup_streams_umount()
 {
 	cd /
 	rm -rf ${SCRATCH_MNT}/stream*
-	umount $SCRATCH_DEV 2>/dev/null
+	_scratch_unmount 2>/dev/null
 }