netfilter: conntrack: Fix gre tunneling over ipv6

This fix permits gre connections to be tracked within ip6tables rules
Signed-off-by: Ludovic Senecaux <linuxludo@free.fr>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_proto_gre.c

@@ -218,9 +218,6 @@ int nf_conntrack_gre_packet(struct nf_conn *ct,
 			    enum ip_conntrack_info ctinfo,
 			    const struct nf_hook_state *state)
 {
-	if (state->pf != NFPROTO_IPV4)
-		return -NF_ACCEPT;
-
 	if (!nf_ct_is_confirmed(ct)) {
 		unsigned int *timeouts = nf_ct_timeout_lookup(ct);