net: dsa: tag_mtk: 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.

Cc: DENG Qingfang <dqfext@gmail.com>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: John Crispin <john@phrozen.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/tag_mtk.c

@@ -34,9 +34,6 @@ static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
 	 * table with VID.
 	 */
 	if (!skb_vlan_tagged(skb)) {
-		if (skb_cow_head(skb, MTK_HDR_LEN) < 0)
-			return NULL;
-
 		skb_push(skb, MTK_HDR_LEN);
 		memmove(skb->data, skb->data + MTK_HDR_LEN, 2 * ETH_ALEN);
 		is_vlan_skb = false;