selftests: mlxsw: Add a RIF counter scale test

This tests creates as many RIFs as possible, ideally more than there can be
RIF counters (though that is currently only possible on Spectrum-1). It
then tries to enable L3 HW stats on each of the RIFs. It also contains the
traffic test, which tries to run traffic through a log2 of those counters
and checks that the traffic is shown in the counter values.

Like with tc_flower traffic test, take a log2 subset of rules. The logic
behind picking log2 rules is that then every bit of the instantiated item's
number is exercised. This should catch issues whether they happen at the
high end, low end, or somewhere in between.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>