netfilter: synproxy: use nf_cookie_v6_check() from core

This helper function is never used and it is intended to avoid a direct
dependency with the ipv6 module.

Fixes: d7f9b2f ("netfilter: synproxy: extract SYNPROXY infrastructure from {ipt, ip6t}_SYNPROXY")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_synproxy_core.c

@@ -1056,7 +1056,7 @@ synproxy_recv_client_ack_ipv6(struct net *net,
  struct synproxy_net *snet = synproxy_pernet(net);
  int mss;
 
- mss = __cookie_v6_check(ipv6_hdr(skb), th, ntohl(th->ack_seq) - 1);
+ mss = nf_cookie_v6_check(ipv6_hdr(skb), th, ntohl(th->ack_seq) - 1);
  if (mss == 0) {
  this_cpu_inc(snet->stats->cookie_invalid);
  return false;