netfilter: check if the socket netns is correct.

Netfilter assumes that if the socket is present in the skb, then
it can be used because that reference is cleaned up while the skb
is crossing netns.

We want to change that to preserve the socket reference in a future
patch, so this is a preparation updating netfilter to check if the
socket netns matches before use it.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>