mlxsw: spectrum_qdisc: Offload action trap for qevents

When offloading action trap on a qevent, pass to_dev of NULL to the SPAN
module to trigger the mirror to the CPU port. Query the buffer drops
policer and use it for policing of the trapped traffic.

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