selftests: mlxsw: vxlan_flooding: Prevent flooding of unwanted packets

The test verifies that packets are correctly flooded by the bridge and
the VXLAN device by matching on the encapsulated packets at the other
end. However, if packets other than those generated by the test also
ingress the bridge (e.g., MLD packets), they will be flooded as well and
interfere with the expected count.

Make the test more robust by making sure that only the packets generated
by the test can ingress the bridge. Drop all the rest using tc filters
on the egress of 'br0' and 'h1'.

In the software data path, the problem can be solved by matching on the
inner destination MAC or dropping unwanted packets at the egress of the
VXLAN device, but this is not currently supported by mlxsw.

Fixes: 94d302de ("selftests: mlxsw: Add a test for VxLAN flooding")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>