odhcp6c: avoid integer overflows in resend logic

git-svn-id: svn://svn.openwrt.org/openwrt@37150 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: odhcp6c: backport r37150

git-svn-id: svn://svn.openwrt.org/openwrt@37151 3c298f89-4303-0410-b956-a3cf2f4a3e73