netfilter: refactor NAT redirect IPv4 to use it from nf_tables

This patch refactors the IPv4 code so it can be usable both from xt and
nf_tables.

A similar patch follows-up to handle IPv6.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>