net_ns: add __rcu annotations

add __rcu annotation to (struct net)->gen, and use
rcu_dereference_protected() in net_assign_generic()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 65af9a0..1bf812b 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -88,7 +88,7 @@
 #ifdef CONFIG_WEXT_CORE
 	struct sk_buff_head	wext_nlevents;
 #endif
-	struct net_generic	*gen;
+	struct net_generic __rcu	*gen;
 
 	/* Note : following structs are cache line aligned */
 #ifdef CONFIG_XFRM