netinet: make ifa_ifwithaddr() and ifa_ifwithaddr_check() fib-aware

[nc]

Summary

netinet: make ifa_ifwithaddr() and ifa_ifwithaddr_check() fib-aware

Submitted by: Neel Chauhan <neel AT neelc DOT org>

Test Plan

1. Set net.fibs between 2-16 in /boot/loader.conf

1. Add a route in say FIB 1 which points to a subnet (e.g. default router, or subnet behind subrouter)

2. Do a ping/traceroute to a destination with an ICMP redirect, say a subrouter behind your router, but the subrouter's "external" interface is on the same subnet but different IP as your fib 1 router IP (which also has a route to the subrouter)

[melifaro]

Can you please fill in the testing section?

[nc]

Sorry for the delay, I filled in the testing section.