netns: add dummy struct inside "struct net_generic"

This is precursor to fixing "[id - 1]" bloat inside net_generic().

Name "s" is chosen to complement name "u" often used for dummy unions.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/generic.h

@@ -25,8 +25,10 @@
  */
 
 struct net_generic {
-	unsigned int len;
-	struct rcu_head rcu;
+	struct {
+		unsigned int len;
+		struct rcu_head rcu;
+	} s;
 
 	void *ptr[0];
 };