Add rib_lookup() sockaddr lookup wrapper and make ifa_ifwithroute use it.

Create rib_lookup() wrapper around per-af dataplane lookup functions.
This will help in the cases of having control plane af-agnostic code.

Switch ifa_ifwithroute() to use this function instead of rtalloc1().

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D24731