odhcp6c: add sanity checks for NDP and MTU parameters from RAs

git-svn-id: svn://svn.openwrt.org/openwrt@36930 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: odhcp6c: backport r36930

git-svn-id: svn://svn.openwrt.org/openwrt@36931 3c298f89-4303-0410-b956-a3cf2f4a3e73