selftests: mlxsw: Add a RED selftest

This tests that below the queue minimum length, there is no dropping /
marking, and above max, everything is dropped / marked.

The test is structured as a core file with topology and test code, and
three wrappers: one for RED used as a root Qdisc, and two for
testing (W)RED under PRIO and ETS.

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

tools/testing/selftests/net/forwarding/lib.sh

@@ -607,6 +607,16 @@ ethtool_stats_get()
 	ethtool -S $dev | grep "^ *$stat:" | head -n 1 | cut -d: -f2
 }
 
+qdisc_stats_get()
+{
+	local dev=$1; shift
+	local handle=$1; shift
+	local selector=$1; shift
+
+	tc -j -s qdisc show dev "$dev" \
+	    | jq '.[] | select(.handle == "'"$handle"'") | '"$selector"
+}
+
 humanize()
 {
 	local speed=$1; shift