[NET]: Protect device namespace inlines with CONFIG_NET

Include sites should not be bothered by whether
CONFIG_NET is set or not when trying to include
benign files like linux/etherdevice.h et al.

From a report by Stephen Rothwell.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -741,6 +741,7 @@ struct net_device
 #define	NETDEV_ALIGN		32
 #define	NETDEV_ALIGN_CONST	(NETDEV_ALIGN - 1)
 
+#ifdef CONFIG_NET
 /*
  * Net namespace inlines
  */
@@ -761,6 +762,7 @@ void dev_net_set(struct net_device *dev, struct net *net)
 	dev->nd_net = net;
 #endif
 }
+#endif
 
 /**
  *	netdev_priv - access network device private data