[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/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
index 96138b1..08e8fb6 100644
--- a/net/ipv4/ipconfig.c
+++ b/net/ipv4/ipconfig.c
@@ -434,7 +434,7 @@
 	unsigned char *sha, *tha;		/* s for "source", t for "target" */
 	struct ic_device *d;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto drop;
 
 	if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)
@@ -854,7 +854,7 @@
 	struct ic_device *d;
 	int len, ext_len;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto drop;
 
 	/* Perform verifications before taking the lock.  */