netfilter: conntrack: don't log "invalid" icmpv6 connections

When enabling logging for invalid connections we currently also log most
icmpv6 types, which we don't track intentionally (e.g. neigh discovery).
"invalid" should really mean "invalid", i.e. short header or bad checksum.

We don't do any logging for icmp(v4) either, its just useless noise.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>