[NETLINK]: Fix ACK processing after netlink_dump_start

Revert to original netlink behavior. Do not reply with ACK if the
netlink dump has bees successfully started.

libnl has been broken by the cd40b7d3
The following command reproduce the problem:
   /nl-route-get 192.168.1.1

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>