Fix rt_ifa selection during loopback route insertion process.
Currently such routes are added with a link-level IFA, which is plain wrong. Only after the insertion they get fixed by the special link_rtrequest() ifa handler. This behaviour complicates routing code and makes ifa selection more complex.
Streamline this process by explicitly moving link_rtrequest() logic
to the pre-insertion rt_getifa_fib() ifa selector. Avoid calling all this logic in the loopback route case by explicitly specifying proper rt_ifa inside the ifa_maintain_loopback_route().ยง
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20076