odhcp6c: Revert to slightly modified version of old behaviour

Latest changes broke source-restriction of on-link routes. Restoring
old-behaviour but adding correct source-restrictions which prevents
the offlink handler to remove the on-link route.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39454 3c298f89-4303-0410-b956-a3cf2f4a3e73