netfilter: nfnetlink: add and use nfnetlink_broadcast

This removes the only reference of net->nfnl outside of the nfnetlink
module.  This allows to move net->nfnl to net_generic infra.

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