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>

tests/xfs/227

@@ -161,7 +161,7 @@ _scratch_mount
 fragment_freespace
 
 # unmount and remount to reset all allocator indexes
-umount $SCRATCH_MNT
+_scratch_unmount
 _scratch_mount
 
 # create a range of source files, then fsr them to a known size
@@ -205,6 +205,6 @@ for n in `seq 4 1 12`; do
 	done
 done
 
-umount $SCRATCH_MNT
+_scratch_unmount
 echo "--- silence is golden ---"
 status=0 ; exit