net: dm9051: Fix use after free in dm9051_loop_tx()

This code dereferences "skb" after calling dev_kfree_skb().

Fixes: 2dc95a4d ("net: Add dm9051 driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20220221105440.GA10045@kiliSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/davicom/dm9051.c

@@ -845,17 +845,19 @@ static int dm9051_loop_tx(struct board_info *db)
 
 	while (!skb_queue_empty(&db->txq)) {
 		struct sk_buff *skb;
+		unsigned int len;
 
 		skb = skb_dequeue(&db->txq);
 		if (skb) {
 			ntx++;
 			ret = dm9051_single_tx(db, skb->data, skb->len);
+			len = skb->len;
 			dev_kfree_skb(skb);
 			if (ret < 0) {
 				db->bc.tx_err_counter++;
 				return 0;
 			}
-			ndev->stats.tx_bytes += skb->len;
+			ndev->stats.tx_bytes += len;
 			ndev->stats.tx_packets++;
 		}