net: dsa: tag_gswip: let DSA core deal with TX reallocation

Now that we have a central TX reallocation procedure that accounts for
the tagger's needed headroom in a generic way, we can remove the
skb_cow_head call.

This one is interesting, the DSA tag is 8 bytes on RX and 4 bytes on TX.
Because DSA is unaware of asymmetrical tag lengths, the overhead/needed
headroom is declared as 8 bytes and therefore 4 bytes larger than it
needs to be. If this becomes a problem, and the GSWIP driver can't be
converted to a uniform header length, we might need to make DSA aware of
separate RX/TX overhead values.

Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

net/dsa/tag_gswip.c

@@ -60,13 +60,8 @@ static struct sk_buff *gswip_tag_xmit(struct sk_buff *skb,
 				      struct net_device *dev)
 {
 	struct dsa_port *dp = dsa_slave_to_port(dev);
-	int err;
 	u8 *gswip_tag;
 
-	err = skb_cow_head(skb, GSWIP_TX_HEADER_LEN);
-	if (err)
-		return NULL;
-
 	skb_push(skb, GSWIP_TX_HEADER_LEN);
 
 	gswip_tag = skb->data;