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().