selftests: vxlan_mdb: Fix failures with old libnet

[ Upstream commit f142552 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>