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

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index d172ec4..f304471 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -314,6 +314,8 @@
 	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;