netfilter: nat: un-export nf_nat_l4proto_unique_tuple

almost all l4proto->unique_tuple implementations just call this helper,
so make ->unique_tuple() optional and call its helper directly if the
l4proto doesn't override it.

This is an intermediate step to get rid of ->unique_tuple completely.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>