net: dsa: update TX path comments to not mention skb_mac_header()

Once commit 6d1ccff627 ("net: reset mac header in dev_start_xmit()")
will be reverted, it will no longer be true that skb->data points at
skb_mac_header(skb) - since the skb->mac_header will not be set - so
stop saying that, and just say that it points to the MAC header.

I've reviewed vlan_insert_tag() and it does not *actually* depend on
skb_mac_header(), so reword that to avoid the confusion.

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.h

@@ -229,7 +229,7 @@ static inline void *dsa_etype_header_pos_rx(struct sk_buff *skb)
 	return skb->data - 2;
 }
 
-/* On TX, skb->data points to skb_mac_header(skb), which means that EtherType
+/* On TX, skb->data points to the MAC header, which means that EtherType
  * header taggers start exactly where the EtherType is (the EtherType is
  * treated as part of the DSA header).
  */

net/dsa/tag_8021q.c

@@ -461,8 +461,8 @@ EXPORT_SYMBOL_GPL(dsa_tag_8021q_unregister);
 struct sk_buff *dsa_8021q_xmit(struct sk_buff *skb, struct net_device *netdev,
 			       u16 tpid, u16 tci)
 {
-	/* skb->data points at skb_mac_header, which
-	 * is fine for vlan_insert_tag.
+	/* skb->data points at the MAC header, which is fine
+	 * for vlan_insert_tag().
 	 */
 	return vlan_insert_tag(skb, htons(tpid), tci);
 }