6relayd: correctly set downstream routes for DHCPv6-PD

git-svn-id: svn://svn.openwrt.org/openwrt@36969 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: odhcp6c: backport r36959

git-svn-id: svn://svn.openwrt.org/openwrt@36970 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: 6relayd: backport r36969

git-svn-id: svn://svn.openwrt.org/openwrt@36971 3c298f89-4303-0410-b956-a3cf2f4a3e73