selftests: mlxsw: qos_mc_aware: Fix mausezahn invocation

Mausezahn does not recognize "own" as a keyword on source IP address. As a
result, the MC stream is not running at all, and therefore no UC
degradation can be observed even in principle.

Fix the invocation, and tighten the test: due to the minimum shaper
configured at the MC TCs, we always expect about 20% degradation. Fail the
test if it is lower.

Fixes: 573363a6 ("selftests: mlxsw: Add qos_lib.sh")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Reported-by: Amit Cohen <amitc@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>