netfilter: conntrack: move sysctl pointer to net_generic infra

No need to keep this in struct net, place it in the net_generic data.
The sysctl pointer is removed from struct net in a followup patch.

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