netfilter: ip6t_NPT: restrict to mangle table

As the translation is stateless, using it in nat table
doesn't work (only initial packet is translated).
filter table OUTPUT works but won't re-route the packet after translation.

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