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/033

@@ -34,7 +34,7 @@ status=1	# failure is the default!
 _cleanup()
 {
     cd /
-    umount $SCRATCH_DEV 2>/dev/null
+    _scratch_unmount 2>/dev/null
     rm -f $tmp.*
 }
 
@@ -101,7 +101,7 @@ src/feature -G $SCRATCH_DEV && \
 	_notrun "GQuota are enabled, test needs controlled sb recovery"
 src/feature -P $SCRATCH_DEV && \
 	_notrun "PQuota are enabled, test needs controlled sb recovery"
-umount $SCRATCH_DEV
+_scratch_unmount
 
 # rootino, rbmino, and rsumino are now set (lets blow em away!)
 _check_root_inos 0