net/sched: cls_flower: merge filter delete/destroy common code

Move common code from fl_delete and fl_detroy to __fl_delete.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>