selftests: mlxsw: devlink_trap_tunnel_ipip6: Add test case for IPv6 decap_error

IPv6 underlay support was added, add test to check that "decap_error" trap
is triggered under the right conditions and that devlink counters increase.

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