[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/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index f9ee844..50972b3 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -1190,7 +1190,7 @@
 	struct ip_tunnel *t = netdev_priv(dev);
 	if (ipv4_is_multicast(t->parms.iph.daddr) && t->mlink) {
 		struct in_device *in_dev;
-		in_dev = inetdev_by_index(dev->nd_net, t->mlink);
+		in_dev = inetdev_by_index(dev_net(dev), t->mlink);
 		if (in_dev) {
 			ip_mc_dec_group(in_dev, t->parms.iph.daddr);
 			in_dev_put(in_dev);