net: Convert nfnl_log_net_ops

These pernet_operations create and destroy /proc entries.
Also, exit method unsets nfulnl_logger. The logger is not
set by default, and it becomes bound via userspace request.
So, they look safe to be made async.

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