selftests: vxlan_mdb: Fix failures with old libnet

Locally generated IP multicast packets (such as the ones used in the
test) do not perform routing and simply egress the bound device.

However, as explained in commit 8bcfb4a ("selftests: forwarding:
Fix failing tests with old libnet"), old versions of libnet (used by
mausezahn) do not use the "SO_BINDTODEVICE" socket option. Specifically,
the library started using the option for IPv6 sockets in version 1.1.6
and for IPv4 sockets in version 1.2. This explains why on Ubuntu - which
uses version 1.1.6 - the IPv4 overlay tests are failing whereas the IPv6
ones are passing.

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

Fixes: 62199e3 ("selftests: net: Add VXLAN MDB test")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/5bb50349-196d-4892-8ed2-f37543aa863f@alu.unizg.hr/
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240325075030.2379513-1-idosch@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>