odhcp6c: improve passthrough filter

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40886 3c298f89-4303-0410-b956-a3cf2f4a3e73
odhcp6c: fix last commit

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40887 3c298f89-4303-0410-b956-a3cf2f4a3e73