[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/ipx/af_ipx.c b/net/ipx/af_ipx.c
index c76a952..81ae873 100644
--- a/net/ipx/af_ipx.c
+++ b/net/ipx/af_ipx.c
@@ -335,7 +335,7 @@
 	struct net_device *dev = ptr;
 	struct ipx_interface *i, *tmp;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		return NOTIFY_DONE;
 
 	if (event != NETDEV_DOWN && event != NETDEV_UP)
@@ -1636,7 +1636,7 @@
 	u16 ipx_pktsize;
 	int rc = 0;
 
-	if (dev->nd_net != &init_net)
+	if (dev_net(dev) != &init_net)
 		goto drop;
 
 	/* Not ours */