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/generic/117

@@ -82,7 +82,7 @@ _require_scratch
 _require_attrs
 
 rm -f $seqres.full
-umount $SCRATCH_DEV >/dev/null 2>&1
+_scratch_unmount >/dev/null 2>&1
 echo "*** MKFS ***"                         >>$seqres.full
 echo ""                                     >>$seqres.full
 _scratch_mkfs                               >>$seqres.full 2>&1 \
@@ -105,7 +105,7 @@ while [ $i -lt $ITERATIONS ]; do
 done
 
 cd /
-umount $SCRATCH_MNT
+_scratch_unmount
 echo 
 echo Checking filesystem