pkt_sched: Update drops stats in act_police

Action police statistics could be misleading because drops are not
shown when expected.

With feedback from: Jamal Hadi Salim <hadi@cyberus.ca>

Reported-by: Pawel Staszewski <pstaszewski@itcare.pl>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Acked-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>