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: d7f9b2f1 ("netfilter: synproxy: extract SYNPROXY infrastructure from {ipt, ip6t}_SYNPROXY")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>