Currently we create link-local route by creating an always-on
IPv6 prefix into prefix list. This prefix is not tied to an link-local ifa.
This leads to the following problems:
* When flushing interface addresses we skip on-link route, leaving fe80::/64 prefix on the interface w/o any IPv6 addresses.
* When creating and removing link-local alias we lose fe80::/64 prefix
Fix this by attaching ifa to the prefix at the initial creation.