Revert "netfilter: move nat hlist_head to nf_conn"

This reverts commit 7c966435 as it is
not working properly.  Please move to 4.9 to get the full fix.

Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>