skb_mac_header() will no longer be available in the TX path when
reverting commit 6d1ccff62780 ("net: reset mac header in
dev_start_xmit()"). As preparation for that, let's use
skb_vlan_eth_hdr() to get to the VLAN header instead, which assumes it's
located at skb->data (assumption which holds true here).
Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>