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/net/core/net_namespace.c b/net/core/net_namespace.c
index c988e68..3f86026 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -42,7 +42,9 @@
 	BUG_ON(!mutex_is_locked(&net_mutex));
 	BUG_ON(id == 0);
 
-	ng = old_ng = net->gen;
+	old_ng = rcu_dereference_protected(net->gen,
+					   lockdep_is_held(&net_mutex));
+	ng = old_ng;
 	if (old_ng->len >= id)
 		goto assign;