netfilter: Use nf_ct_net instead of dev_net(out) in nf_nat_masquerade_ipv6

Use nf_ct_net(ct) instead of guessing that the netdevice out can
reliably report the network namespace the conntrack operation is
happening in.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>