mlxsw: spectrum_span: Support mirror-to-VLAN

Offload "tc action mirred mirror" to a device that is a vlan device on
top of a front-panel port device. The hardware encapsulates the mirrored
packets in a VLAN tag. That includes the case that the mirrored traffic
is already VLAN-tagged--in that case the monitor traffic will be
double-tagged, just like in the software path.

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>