[NET_SCHED]: sch_atm: act_api support

Handle act_api classification results.

The ATM scheduler behaves slightly different than other schedulers
in that it only handles policer results for successful classifications,
this behaviour is retained for the act_api case.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>