net: Convert nfnl_acct_ops

These pernet_operations look closed in themself,
and there are no other users of net::nfnl_acct_list
outside. They are safe to be executed for several
net namespaces in parallel.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>