Similar to skb_eth_hdr() introduced in commit 96cc4b69581d ("macvlan: do
not assume mac_header is set in macvlan_broadcast()"), let's introduce a
skb_vlan_eth_hdr() helper which can be used in TX-only code paths to get
to the VLAN header based on skb->data rather than based on the
skb_mac_header(skb).
We also consolidate the drivers that dereference skb->data to go through
this helper.
Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>