datapath: backport: vlan: Introduce helper functions to check if skb is tagged

datapath: backport: vlan: Introduce helper functions to check if skb is tagged

Upstream commit:
    commit f5a7fb88e1f82542ca14ba93a1d4fa35471c60ca
    Author: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
    Date:   Fri Mar 27 14:31:11 2015 +0900

    vlan: Introduce helper functions to check if skb is tagged

    Separate the two checks for single vlan and multiple vlans in
    netif_skb_features().  This allows us to move the check for multiple
    vlans to another function later.

    Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yi Yang <yi.y.yang@intel.com>
Acked-by: Eric Garver <e@erig.me>
Signed-off-by: Joe Stringer <joe@ovn.org>