mlxsw: spectrum_qdisc: Offload mirroring on RED qevent early_drop

The RED qevents early_drop and mark can be offloaded under the following
fairly strict conditions:

- At most one filter is configured at the qevent block
- The protocol is "any"
- The classifier is matchall
- The action is trap, sample, or mirror with the same conditions as
  with other SPAN offloads
- The hw_counters type is none

In this patchset, implement offload of mirror for early_drop qevent.
The ECN trigger is currently not implemented in the FW and therefore
the mark qevent is not supported.

The qevent notifications look exactly like regular block binding
notifications with a binder type that identifies them as qevents.
Therefore the details of processing this binding are fairly similar
to the matchall offload.

struct flow_block_offload.sch points at the qdisc in question. Use it to
figure out if the qdisc is offloaded at all and what TC it configures.
Bounce bindings on not-offloaded qdiscs.

Individual bindings are kept in a list so that several qevents can share
the same block and all binding points get configured as the configured
filters change.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>