routing: add rib_match_gw() helper
Finish 02e05b8faec1:
- add gateway matcher function that can be used in rib_del_route_px() or any rib_walk-family functions. It will be used in the upcoming migration to the new KPI
- rename gw_fulter_func to match_gw_one() to better signal the function purpose / semantic.
MFC after: 1 month