Map IPv6 link-local prefix to the link-local ifa.

Currently we create link-local route by creating an always-on IPv6 prefix
in the prefix list. This prefix is not tied to the link-local ifa.

This leads to the following problems:

First, when flushing interface addresses we skip on-link route, leaving
fe80::/64 prefix on the interface without any IPv6 addresses.
Second, when creating and removing link-local alias we lose fe80::/64 prefix
from the routing table.

Fix this by attaching link-local prefix to the ifa at the initial creation.

Reviewed by: hrs
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D28129