drivers: net: xgene: fix possible use after free

Once TX has been enabled on a NIC, it is illegal to access skb,
as this skb might have been freed by another cpu, from TX completion
handler.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NMark Rutland <mark.rutland@arm.com>
Tested-by: NMark Rutland <mark.rutland@arm.com>
Cc: Iyappan Subramanian <isubramanian@apm.com>
Acked-by: NIyappan Subramanian <isubramanian@apm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/apm/xgene/xgene_enet_main.c

@@ -450,12 +450,12 @@ static netdev_tx_t xgene_enet_start_xmit(struct sk_buff *skb,
 		return NETDEV_TX_OK;
 	}
 
-	pdata->ring_ops->wr_cmd(tx_ring, count);
 	skb_tx_timestamp(skb);
 
 	pdata->stats.tx_packets++;
 	pdata->stats.tx_bytes += skb->len;
 
+	pdata->ring_ops->wr_cmd(tx_ring, count);
 	return NETDEV_TX_OK;
 }