[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/appletalk/aarp.c b/net/appletalk/aarp.c
index 61166f6..25aa37c 100644
--- a/net/appletalk/aarp.c
+++ b/net/appletalk/aarp.c
@@ -333,7 +333,7 @@
 	struct net_device *dev = ptr;
 	int ct;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		return NOTIFY_DONE;
 
 	if (event == NETDEV_DOWN) {
@@ -716,7 +716,7 @@
 	struct atalk_addr sa, *ma, da;
 	struct atalk_iface *ifa;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto out0;
 
 	/* We only do Ethernet SNAP AARP. */