ifa_ifwithroute() is the only magic place where an AF_LINK sockaddr

ifa_ifwithroute() is the only magic place where an AF_LINK sockaddr
can be given to ifa_ifwithnet().

Handle this specific case directly and let ifa_ifwithnet() do only
one thing:  iterate on all the addresses of all the interfaces in a
given routing domain to return the most specific matching address.

ok mikeb@