[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/x25/x25_dev.c b/net/x25/x25_dev.c
index f0679d2..3ff206c 100644
--- a/net/x25/x25_dev.c
+++ b/net/x25/x25_dev.c
@@ -95,7 +95,7 @@
 	struct sk_buff *nskb;
 	struct x25_neigh *nb;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto drop;
 
 	nskb = skb_copy(skb, GFP_ATOMIC);