netfilter: conntrack: avoid using ->error callback if possible

netfilter: conntrack: avoid using ->error callback if possible

The error() handler gets called before allocating or looking up a
connection tracking entry.

We can instead use direct calls from the ->packet() handlers which get
invoked for every packet anyway.

Only exceptions are icmp and icmpv6, these two special cases will be
handled in the next patch.

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