netfilter: cttimeout: use net_generic infra

reduce size of struct net and make this self-contained.
The member in struct net is kept to minimize changes to struct net
layout, it will be removed in a separate patch.

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