This is to simplify using double tagged vlans. This function allows all
valid vlan ethertypes to be checked in a single function call.
Also replace some instances that check for both ETH_P_8021Q and
ETH_P_8021AD.

Patch based on one originally by Thomas F Herbert.
Signed-off-by: NThomas F Herbert <thomasfherbert@gmail.com>
Signed-off-by: NEric Garver <e@erig.me>
Acked-by: NPravin B Shelar <pshelar@ovn.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>