selftests: mlxsw: sch_red_ets: Test proper counter cleaning in ETS

There was a bug introduced during the rework which cause non-zero backlog
being stuck at ETS. Introduce a selftest that would have caught the issue
earlier.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/drivers/net/mlxsw/sch_red_ets.sh

@@ -67,6 +67,13 @@ red_test()
 {
 	install_qdisc
 
+	# Make sure that we get the non-zero value if there is any.
+	local cur=$(busywait 1100 until_counter_is "> 0" \
+			    qdisc_stats_get $swp3 10: .backlog)
+	(( cur == 0 ))
+	check_err $? "backlog of $cur observed on non-busy qdisc"
+	log_test "$QDISC backlog properly cleaned"
+
 	do_red_test 10 $BACKLOG1
 	do_red_test 11 $BACKLOG2