net: dsa: tag_dsa: 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.

Similar to the EtherType DSA tagger, the old Marvell tagger can
transform an 802.1Q header if present into a DSA tag, so there is no
headroom required in that case. But we are ensuring that it exists,
regardless (practically speaking, the headroom must be 4 bytes larger
than it needs to be).
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_dsa.c

@@ -23,9 +23,6 @@ static struct sk_buff *dsa_xmit(struct sk_buff *skb, struct net_device *dev)
 	 * the ethertype field for untagged packets.
 	 */
 	if (skb->protocol == htons(ETH_P_8021Q)) {
-		if (skb_cow_head(skb, 0) < 0)
-			return NULL;
-
 		/*
 		 * Construct tagged FROM_CPU DSA tag from 802.1q tag.
 		 */
@@ -41,8 +38,6 @@ static struct sk_buff *dsa_xmit(struct sk_buff *skb, struct net_device *dev)
 			dsa_header[2] &= ~0x10;
 		}
 	} else {
-		if (skb_cow_head(skb, DSA_HLEN) < 0)
-			return NULL;
 		skb_push(skb, DSA_HLEN);
 
 		memmove(skb->data, skb->data + DSA_HLEN, 2 * ETH_ALEN);