[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/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index fb37b80..629c909 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -91,7 +91,7 @@
 	int len, err;
 	struct lapbethdev *lapbeth;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto drop;
 
 	if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)
@@ -393,7 +393,7 @@
 	struct lapbethdev *lapbeth;
 	struct net_device *dev = ptr;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		return NOTIFY_DONE;
 
 	if (!dev_is_ethdev(dev))