[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>

drivers/net/loopback.c

@@ -258,7 +258,7 @@ static __net_init int loopback_net_init(struct net *net)
 	if (!dev)
 		goto out;
 
-	dev->nd_net = net;
+	dev_net_set(dev, net);
 	err = register_netdev(dev);
 	if (err)
 		goto out_free_netdev;