ip_tunnel: Initialize the fallback device properly

We need to initialize the fallback device to have a correct mtu
set on this device. Otherwise the mtu is set to null and the device
is unusable.

Fixes: fd58156e ("IPIP: Use ip-tunneling code.")
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -883,6 +883,7 @@ int ip_tunnel_init_net(struct net *net, int ip_tnl_net_id,
 	 */
 	if (!IS_ERR(itn->fb_tunnel_dev)) {
 		itn->fb_tunnel_dev->features |= NETIF_F_NETNS_LOCAL;
+		itn->fb_tunnel_dev->mtu = ip_tunnel_bind_dev(itn->fb_tunnel_dev);
 		ip_tunnel_add(itn, netdev_priv(itn->fb_tunnel_dev));
 	}
 	rtnl_unlock();