common/rc: introduce _overlay_mount_dirs helper

common/rc: introduce _overlay_mount_dirs helper

Introduce a new _overlay_mount_dirs() helper to do the actual
overlay mount operation and factor _overlay_scratch_mount and
_overlay_mount code to use a single implemention of
_overlay_mount_dirs(). Also convert all bare mount of overlay in
tests to use it.

Suggested-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>