netfilter: nf_conntrack_ipv6: don't track ICMPv6 negotiation message

This patch removes connection tracking handling for ICMPv6 messages
related to Stateless Address Autoconfiguration, MLD, and MLDv2. They
can not be tracked because they are massively using multicast (on
pre-defined address). But they are not invalid and should not be
detected as such.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>