[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/wireless/wext.c b/net/wireless/wext.c
index 2c569b6..947188a 100644
--- a/net/wireless/wext.c
+++ b/net/wireless/wext.c
@@ -1157,7 +1157,7 @@
 	struct sk_buff *skb;
 	int err;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		return;
 
 	skb = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_ATOMIC);