vlan: Check for vlan ethernet types for 8021.q or 802.1ad

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: Thomas F Herbert <thomasfherbert@gmail.com>
Signed-off-by: Eric Garver <e@erig.me>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>