mlxsw: spectrum_span: Support VLAN under mirror-to-gretap

When mirroring to a gretap or ip6gretap device, allow the underlay
packet path to include VLAN devices. The following configurations are
supported in underlay:

- vlan over phys
- vlan-unaware bridge where the egress device is vlan over phys
- vlan over vlan-aware bridge where the egress device is phys

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