net: dsa: tag_sja1105: don't rely on skb_mac_header() in TX paths

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_sja1105.c

@@ -256,7 +256,7 @@ static struct sk_buff *sja1105_pvid_tag_control_pkt(struct dsa_port *dp,
 			return NULL;
 	}
 
-	hdr = (struct vlan_ethhdr *)skb_mac_header(skb);
+	hdr = skb_vlan_eth_hdr(skb);
 
 	/* If skb is already VLAN-tagged, leave that VLAN ID in place */
 	if (hdr->h_vlan_proto == xmit_tpid)