netlink: Fix adding routes with nexthops on p2p interfaces.

Use full-featured ifa_ifwithroute() to guess route ifa/ifp
instead of ifa_ifwithnet(). This change makes the route addition
logic closer to the rt_getifa_fib() used by rtsock.

Reported by: glebius
Tested by: glebius
Differential Revision: https://reviews.freebsd.org/D39335
MFC after: 2 weeks