netfilter: nat: remove module dependency on ipv6 core

nf_nat_ipv6 calls two ipv6 core functions, so add those to v6ops to avoid
the module dependency.

This is a prerequisite for merging ipv4 and ipv6 nat implementations.

Add wrappers to avoid the indirection if ipv6 is builtin.

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