mlxsw: spectrum_matchall: Add support for egress sampling

Allow user space to install a matchall classifier with sample action on
egress. This is only supported on Spectrum-2 onwards, so Spectrum-1 will
continue to return an error.

Programming the hardware to sample on egress is identical to ingress
sampling with the sole change of using a different sampling trigger.

Upon receiving a sampled packet, the sampling trigger (ingress vs.
egress) will be encoded in the mirroring reason in the Completion Queue
Element (CQE). The mirroring reason is used to lookup the sampling
parameters (e.g., psample group) which are passed to the psample module.

Note that locally generated packets that are sampled are simply
consumed. This is done for several reasons.

First, such packets do not have an ingress netdev given that their Rx
local port is the CPU port. This breaks several basic assumptions.

Second, sampling using the same interface (tc), but with flower
classifier will not result in locally generated packets being sampled
given that such packets are not subject to the policy engine.

Third, realistically, this is not a big deal given that the vast
majority of the packets being transmitted through the port are not
locally generated packets.

Fourth, if such packets do need to be sampled, they can be sampled with
a 'skip_hw' filter and reported to the same sampling group as the data
path packets. The software sampling rate can also be adjusted to fit the
rate of the locally generated packets which is much lower than the rate
of the data path traffic.

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