net_sched: allow flushing tc police actions

The act_police uses its own code to walk the
action hashtable, which leads to that we could
not flush standalone tc police actions, so just
switch to tcf_generic_walker() like other actions.

(Joint work from Roman and Cong.)

Signed-off-by: Roman Mashak <mrv@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>