[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>
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 2bd9c5f..09250a0 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -972,7 +972,7 @@
 			goto err_free;
 	}
 
-	dev->nd_net = net;
+	dev_net_set(dev, net);
 	dev->rtnl_link_ops = ops;
 
 	if (tb[IFLA_MTU])
@@ -1198,7 +1198,7 @@
 
 void rtmsg_ifinfo(int type, struct net_device *dev, unsigned change)
 {
-	struct net *net = dev->nd_net;
+	struct net *net = dev_net(dev);
 	struct sk_buff *skb;
 	int err = -ENOBUFS;