netifd: Satisfy IPv6 assignments ordered by prefix length

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36950 3c298f89-4303-0410-b956-a3cf2f4a3e73
netifd: IPv6: Fix sorting order in last commit.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36952 3c298f89-4303-0410-b956-a3cf2f4a3e73