netfilter: nf_tables: fix register ordering

We must register nfnetlink ops last, as that exposes nf_tables to
userspace.  Without this, we could theoretically get nfnetlink request
before net->nft state has been initialized.

Fixes: 99633ab2 ("netfilter: nf_tables: complete net namespace support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>