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/082

@@ -41,7 +41,7 @@ _cleanup()
     cd /
     _cleanup_logfiles
     rm -f $tmp.*
-    umount $SCRATCH_MNT 2>/dev/null
+    _scratch_unmount 2>/dev/null
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15
 
@@ -58,7 +58,7 @@ _require_v2log
 _link_out_file_named $seqfull.op $(_get_os_name)
 
 echo "*** init FS"
-umount $SCRATCH_DEV >/dev/null 2>&1
+_scratch_unmount >/dev/null 2>&1
 
 cat >$tmp.seq.params <<EOF
 # mkfs-opt             mount-opt            start-blk