netfilter: nfnetlink_queue: provide rcu enabled callbacks

nenetlink_queue operations on SMP are not efficent if several queues are
used, because of nfnl_mutex contention when applications give packet
verdict.

Use new call_rcu field in struct nfnl_callback to advertize a callback
that is called under rcu_read_lock instead of nfnl_mutex.

On my 2x4x2 machine, I was able to reach 2.000.000 pps going through
user land returning NF_ACCEPT verdicts without losses, instead of less
than 500.000 pps before patch.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Florian Westphal <fw@strlen.de>
CC: Eric Leblond <eric@regit.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>