netfilter: xt_NFQUEUE: coalesce IPv4 and IPv6 hashing

Because rev1 and rev3 of the target share the same hashing
generalize it by introduing nfqueue_hash().

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>