selftests: forwarding: bridge_mdb_max: Fix failing test with old libnet

As explained in commit 8bcfb4ae ("selftests: forwarding: Fix failing
tests with old libnet"), old versions of libnet (used by mausezahn) do
not use the "SO_BINDTODEVICE" socket option. For IP unicast packets,
this can be solved by prefixing mausezahn invocations with "ip vrf
exec". However, IP multicast packets do not perform routing and simply
egress the bound device, which does not exist in this case.

Fix by specifying the source and destination MAC of the packet which
will cause mausezahn to use a packet socket instead of an IP socket.

Fixes: 3446dcd7

 ("selftests: forwarding: bridge_mdb_max: Add a new selftest")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/adc5e40d-d040-a65e-eb26-edf47dac5b02@alu.unizg.hr/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-17-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>