netns xfrm: add struct xfrm_state::xs_net

To avoid unnecessary complications with passing netns around.

* set once, very early after allocating
* once set, never changes

For a while create every xfrm_state in init_net.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipcomp.c

@@ -49,7 +49,7 @@ static struct xfrm_state *ipcomp_tunnel_create(struct xfrm_state *x)
 {
 	struct xfrm_state *t;
 
-	t = xfrm_state_alloc();
+	t = xfrm_state_alloc(&init_net);
 	if (t == NULL)
 		goto out;