[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>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 3b54f8a..8576ca9 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -741,6 +741,7 @@
 #define	NETDEV_ALIGN		32
 #define	NETDEV_ALIGN_CONST	(NETDEV_ALIGN - 1)
 
+#ifdef CONFIG_NET
 /*
  * Net namespace inlines
  */
@@ -761,6 +762,7 @@
 	dev->nd_net = net;
 #endif
 }
+#endif
 
 /**
  *	netdev_priv - access network device private data