selftests: mlxsw: Add a test for VxLAN flooding

The device stores flood records in a singly linked list where each
record stores up to three IPv4 addresses of remote VTEPs. The test
verifies that packets are correctly flooded in various cases such as
deletion of a record in the middle of the list.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>