net: dsa: tag_ocelot: do not rely on skb_mac_header() for VLAN xmit

skb_mac_header() will no longer be available in the TX path when
reverting commit 6d1ccff6 ("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@nxp.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_ocelot.c

@@ -26,7 +26,7 @@ static void ocelot_xmit_get_vlan_info(struct sk_buff *skb, struct dsa_port *dp,
 		return;
 	}
 
-	hdr = (struct vlan_ethhdr *)skb_mac_header(skb);
+	hdr = skb_vlan_eth_hdr(skb);
 	br_vlan_get_proto(br, &proto);
 
 	if (ntohs(hdr->h_vlan_proto) == proto) {