netfilter: xt_socket: use IP early demux

With IP early demux added in linux-3.6, we perform TCP lookup in IP
layer before iptables hooks.

We can avoid doing a second lookup in xt_socket.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>