[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/veth.c b/drivers/net/veth.c
index e2ad98b..31cd817 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -375,7 +375,7 @@
 	else
 		snprintf(ifname, IFNAMSIZ, DRV_NAME "%%d");
 
-	peer = rtnl_create_link(dev->nd_net, ifname, &veth_link_ops, tbp);
+	peer = rtnl_create_link(dev_net(dev), ifname, &veth_link_ops, tbp);
 	if (IS_ERR(peer))
 		return PTR_ERR(peer);