sched: cls_flower: expose priority to offloading netdevice

The driver that offloads flower rules needs to know with which priority
user inserted the rules. So add this information into offload struct.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>