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/041

@@ -35,7 +35,7 @@ status=1	# failure is the default!
 _cleanup()
 {
     cd /
-    umount $SCRATCH_MNT
+    _scratch_unmount
     rm -f $tmp.*
 }
 trap "_cleanup ; exit \$status" 0 1 2 3 15
@@ -50,7 +50,7 @@ _supported_os IRIX Linux
 
 _require_scratch
 _require_no_large_scratch_dev
-umount $SCRATCH_DEV 2>/dev/null
+_scratch_unmount 2>/dev/null
 
 _fill()
 {
@@ -80,7 +80,7 @@ do
     _fill $SCRATCH_MNT/fill_$size
     _do "Grow filesystem to ${size}m" "xfs_growfs -D $grow_size $SCRATCH_MNT"
     echo -n "Flush filesystem... "
-    _do "umount $SCRATCH_MNT"
+    _do "_scratch_unmount"
     _do "_scratch_mount"
     echo "done"
     echo -n "Check files... "