ixgbe: support skb->xmit_more in netdev_ops->ndo_start_xmit()

This implements the deferred tail pointer flush API for the ixgbe
driver. Similar version also proposed longer time ago by Alexander Duyck.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -6958,9 +6958,10 @@ static void ixgbe_tx_map(struct ixgbe_ring *tx_ring,
 
 	tx_ring->next_to_use = i;
 
+	if (!skb->xmit_more) {
 		/* notify HW of packet */
 		ixgbe_write_tail(tx_ring, i);
-
+	}
 	return;
 dma_error:
 	dev_err(tx_ring->dev, "TX DMA map failed\n");