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/299

@@ -40,7 +40,7 @@ status=1	# failure is the default!
 _cleanup()
 {
 	cd /
-	umount $SCRATCH_MNT 2>/dev/null
+	_scratch_unmount 2>/dev/null
 	rm -f $tmp.*
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15
@@ -225,7 +225,7 @@ _exercise g
 _exercise u
 
 echo "*** unmount"
-umount $SCRATCH_MNT
+_scratch_unmount
 
 
 _scratch_mkfs_xfs -m crc=1 2>/dev/null | _filter_mkfs 2>$tmp.mkfs
@@ -241,7 +241,7 @@ _exercise gno
 _exercise pno
 
 echo "*** unmount"
-umount $SCRATCH_MNT
+_scratch_unmount
 
 # success, all done
 status=0