netns: remove one sparse warning

net/core/net_namespace.c:227:18: warning: incorrect type in argument 1
(different address spaces)
net/core/net_namespace.c:227:18:    expected void const *<noident>
net/core/net_namespace.c:227:18:    got struct net_generic [noderef]
<asn:4>*gen

We can use rcu_access_pointer() here as read-side access to the pointer
was removed at least one grace period ago.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -224,7 +224,7 @@ static void net_free(struct net *net)
 		return;
 	}
 #endif
-	kfree(net->gen);
+	kfree(rcu_access_pointer(net->gen));
 	kmem_cache_free(net_cachep, net);
 }