Fix rt_ifa selection during loopback route insertion process.

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().§

MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20076