selftests: mlxsw: spectrum-2: Add a test for VxLAN flooding with IPv6

The device stores flood records in a singly linked list where each
record stores up to X IP addresses of remote VTEPs.

The number of records is changed according to ASIC type and address
family.

Add a test which is similar to the existing IPv4 test to check IPv6.
The test is dedicated for Spectrum-2 and above, which support up to four
IPv6 addresses in one record.

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: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>