[Cake] Hopefully fixed ACK filter for v6

[Toke Høiland-Jørgensen]

Think I finally managed to fix the ACK filter so it works for both 6in4
and regular v6 (the latter seems to have been broken since the commit
"e6b72c2 ack_filter: make less aggressive by default" which accidentally
reversed the address compare logic). It should also be safe against
weirdly fragmented packets (ha! famous last words!).

If someone wants to either eyeball the code and/or test the ACK filter
please do. I'm planning to resubmit the patch to netdev sometime
tomorrow evening :)

-Toke