ether: Add dedicated Ethertype for pseudo-802.1Q DSA tagging

There are two possible utilizations so far:

- Switch devices that don't support a native insertion/extraction header
  on the CPU port may still enjoy the benefits of port isolation with a
  custom VLAN tag.

  For this, they need to have a customizable TPID in hardware and a new
  Ethertype to distinguish between real 802.1Q traffic and the private
  tags used for port separation.

- Switches that don't support the deactivation of VLAN awareness, but
  still want to have a mode in which they accept all traffic, including
  frames that are tagged with a VLAN not configured on their ports, may
  use this as a fake to trick the hardware into thinking that the TPID
  for VLAN is something other than 0x8100.

What follows after the ETH_P_DSA_8021Q EtherType is a regular VLAN
header (TCI), however there is no other EtherType that can be used for
this purpose and doesn't already have a well-defined meaning.
ETH_P_8021AD, ETH_P_QINQ1, ETH_P_QINQ2 and ETH_P_QINQ3 expect that
another follow-up VLAN tag is present, which is not the case here.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>