net: broadcom: bcm4908_enet: update TX stats after actual transmission

Queueing packets doesn't guarantee their transmission. Update TX stats
after hardware confirms consuming submitted data.

This also fixes a possible race and NULL dereference.
bcm4908_enet_start_xmit() could try to access skb after freeing it in
the bcm4908_enet_poll_tx().
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Fixes: 4feffead ("net: broadcom: bcm4908enet: add BCM4908 controller driver")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20221027112430.8696-1-zajec5@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/bcm4908_enet.c

@@ -561,8 +561,6 @@ static netdev_tx_t bcm4908_enet_start_xmit(struct sk_buff *skb, struct net_devic
 
 	if (++ring->write_idx == ring->length - 1)
 		ring->write_idx = 0;
-	enet->netdev->stats.tx_bytes += skb->len;
-	enet->netdev->stats.tx_packets++;
 
 	return NETDEV_TX_OK;
 }
@@ -635,6 +633,7 @@ static int bcm4908_enet_poll_tx(struct napi_struct *napi, int weight)
 	struct bcm4908_enet_dma_ring_bd *buf_desc;
 	struct bcm4908_enet_dma_ring_slot *slot;
 	struct device *dev = enet->dev;
+	unsigned int bytes = 0;
 	int handled = 0;
 
 	while (handled < weight && tx_ring->read_idx != tx_ring->write_idx) {
@@ -645,12 +644,17 @@ static int bcm4908_enet_poll_tx(struct napi_struct *napi, int weight)
 
 		dma_unmap_single(dev, slot->dma_addr, slot->len, DMA_TO_DEVICE);
 		dev_kfree_skb(slot->skb);
-		if (++tx_ring->read_idx == tx_ring->length)
-			tx_ring->read_idx = 0;
 
 		handled++;
+		bytes += slot->len;
+
+		if (++tx_ring->read_idx == tx_ring->length)
+			tx_ring->read_idx = 0;
 	}
 
+	enet->netdev->stats.tx_packets += handled;
+	enet->netdev->stats.tx_bytes += bytes;
+
 	if (handled < weight) {
 		napi_complete_done(napi, handled);
 		bcm4908_enet_dma_ring_intrs_on(enet, tx_ring);