netfilter: iptables: Fix potential null-ptr-deref in ip6table_nat_table_init().

In the Linux kernel, the following vulnerability has been resolved: netfilter: iptables: Fix potential null-ptr-deref in ip6table_nat_table_init(). ip6table_nat_table_init() accesses net->gen->ptr[ip6table_nat_net_ops.id], but the function is exposed to user space before the entry is allocated via register_pernet_subsys(). Let's call register_pernet_subsys() before xt_register_template().