net/if.c: Add fib-aware ifa_ifwithaddr()

Add FIB selection logic by introducing ifa_ifwithaddr_fib() to
support FIB-specific lookups. Then have ifa_ifwithaddr() wrap it
with RT_ALL_FIBS.
Also, do the same for ifa_ifwithaddr_check().

Reviewed by: glebius, bnovkov
Differential Revision: https://reviews.freebsd.org/D58305