net: add 802.1ad / 802.1ah / QinQ ethertypes

define ETH_P_8021AD to 88a8 (assigned by IEEE) and add ETH_P_QINQ{1,2,3}
for the pre-standard 9{1,2,3}00 types. all of them use 802.1q frame
format, with 1 bit used differently in some cases.

also define ETH_P_8021AH to 88e7 (assigned by IEEE). this is Mac-in-Mac
and uses a different, 16-byte header.

Signed-off-by: David Lamparter <equinox@diac24.net>
Signed-off-by: David S. Miller <davem@davemloft.net>