[NET] NETNS: Omit net_device->nd_net without CONFIG_NET_NS.

Introduce per-net_device inlines: dev_net(), dev_net_set().
Without CONFIG_NET_NS, no namespace other than &init_net exists.
Let's explicitly define them to help compiler optimizations.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ax25/ax25_in.c

@@ -451,7 +451,7 @@ int ax25_kiss_rcv(struct sk_buff *skb, struct net_device *dev,
 	skb->sk = NULL;		/* Initially we don't know who it's for */
 	skb->destructor = NULL;	/* Who initializes this, dammit?! */
 
-	if (dev->nd_net != &init_net) {
+	if (dev_net(dev) != &init_net) {
 		kfree_skb(skb);
 		return 0;
 	}