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

(cherry picked from commit 730bfa28057405b44a0b55a4dc35cdc42144ff6d)