[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/pppoe.c b/drivers/net/pppoe.c
index ac0ac98..4fad4dd 100644
--- a/drivers/net/pppoe.c
+++ b/drivers/net/pppoe.c
@@ -301,7 +301,7 @@
 {
 	struct net_device *dev = (struct net_device *) ptr;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		return NOTIFY_DONE;
 
 	/* Only look at sockets that are using this specific device. */
@@ -392,7 +392,7 @@
 	if (!(skb = skb_share_check(skb, GFP_ATOMIC)))
 		goto out;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto drop;
 
 	if (!pskb_may_pull(skb, sizeof(struct pppoe_hdr)))
@@ -424,7 +424,7 @@
 	struct pppoe_hdr *ph;
 	struct pppox_sock *po;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto abort;
 
 	if (!pskb_may_pull(skb, sizeof(struct pppoe_hdr)))