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().

Fixes: fdacd57c

 ("netfilter: x_tables: never register tables by default")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>