selftests: mlxsw: qos_lib: Add a wrapper for running mlnx_qos

mlnx_qos is a script for configuration of DCB. Despite the name it is not
actually Mellanox-specific in any way. It is currently the only ad-hoc tool
available (in contrast to a daemon that manages an interface on an ongoing
basis). However, it is very verbose and parsing out error messages is not
really possible. Add a wrapper that makes it easier to use the tool.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -82,3 +82,17 @@ bail_on_lldpad()
 		fi
 	fi
 }
+
+__mlnx_qos()
+{
+	local err
+
+	mlnx_qos "$@" 2>/dev/null
+	err=$?
+
+	if ((err)); then
+		echo "Error ($err) in mlnx_qos $@" >/dev/stderr
+	fi
+
+	return $err
+}