gst/gstelement.c: Use GST_PAD_ macros instead of GST_RPAD_, since we don't know if it's a real or ghost pad.

Original commit message from CVS:
* gst/gstelement.c: (gst_element_get_compatible_pad_filtered),
(gst_element_link_pads_filtered): Use GST_PAD_ macros instead
of GST_RPAD_, since we don't know if it's a real or ghost pad.