[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/dst.c b/net/core/dst.c
index 3a01a81..694cd2a 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -279,7 +279,7 @@
 	if (!unregister) {
 		dst->input = dst->output = dst_discard;
 	} else {
-		dst->dev = dst->dev->nd_net->loopback_dev;
+		dst->dev = dev_net(dst->dev)->loopback_dev;
 		dev_hold(dst->dev);
 		dev_put(dev);
 		if (dst->neighbour && dst->neighbour->dev == dev) {