selftests: forwarding: ETS: Use Qdisc counters

Currently the SW-datapath ETS selftests use "ip link" stats to obtain the
number of packets that went through a given band. mlxsw then uses ethtool
per-priority counters.

Instead, change both to use qdiscs. In SW datapath this is the obvious
choice, and now that mlxsw offloads FIFO, this should work on the offloaded
datapath as well. This has the effect of verifying that the FIFO offload
works.

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