ip6_gre: Return an error when adding an existing tunnel.

ip6gre_tunnel_locate() should not return an existing tunnel if
create is true. Otherwise it is possible to add the same
tunnel multiple times without getting an error.

So return NULL if the tunnel that should be created already
exists.
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -314,6 +314,8 @@ static struct ip6_tnl *ip6gre_tunnel_locate(struct net *net,
 	struct ip6gre_net *ign = net_generic(net, ip6gre_net_id);
 
 	t = ip6gre_tunnel_find(net, parms, ARPHRD_IP6GRE);
+	if (t && create)
+		return NULL;
 	if (t || !create)
 		return t;