routing: refactor #2

• Use same filter func (rib_filter_f_t) for nexhtop groups to simplify callbacks.
• simplify conditional route deletion & remove the need to pass rt_addrinfo to the low-level deletion functions
• speedup rib_walk_del() by removing an additional per-prefix lookup

Differential Revision: https://reviews.freebsd.org/D36071
MFC after: 1 month