ip_gre: fix fallback tunnel setup

Before making the fallback tunnel visible to lookups, we should make
sure it is completely setup, once ipgre_tunnel_init() had been called
and tstats per_cpu pointer allocated.

move rcu_assign_pointer(ign->tunnels_wc[0], tunnel); from
ipgre_fb_tunnel_init() to ipgre_init_net()

Based on a patch from Pavel Emelyanov
Reported-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Acked-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -1325,7 +1325,6 @@ static void ipgre_fb_tunnel_init(struct net_device *dev)
 {
 	struct ip_tunnel *tunnel = netdev_priv(dev);
 	struct iphdr *iph = &tunnel->parms.iph;
-	struct ipgre_net *ign = net_generic(dev_net(dev), ipgre_net_id);
 
 	tunnel->dev = dev;
 	strcpy(tunnel->parms.name, dev->name);
@@ -1336,7 +1335,6 @@ static void ipgre_fb_tunnel_init(struct net_device *dev)
 	tunnel->hlen		= sizeof(struct iphdr) + 4;
 
 	dev_hold(dev);
-	rcu_assign_pointer(ign->tunnels_wc[0], tunnel);
 }
 
 
@@ -1383,10 +1381,12 @@ static int __net_init ipgre_init_net(struct net *net)
 	if ((err = register_netdev(ign->fb_tunnel_dev)))
 		goto err_reg_dev;
 
+	rcu_assign_pointer(ign->tunnels_wc[0],
+			   netdev_priv(ign->fb_tunnel_dev));
 	return 0;
 
 err_reg_dev:
-	free_netdev(ign->fb_tunnel_dev);
+	ipgre_dev_free(ign->fb_tunnel_dev);
 err_alloc_dev:
 	return err;
 }