net: ena: don't wake up tx queue when down

There is a race condition that can occur when calling ena_down().
The ena_clean_tx_irq() - which is a part of the napi handler -
function might wake up the tx queue when the queue is supposed
to be down (during recovery or changing the size of the queues
for example) This causes the ena_start_xmit() function to trigger
and possibly try to access the destroyed queues.

The race is illustrated below:

Flow A:                                       Flow B(napi handler)
ena_down()
   netif_carrier_off()
   netif_tx_disable()
                                                      ena_clean_tx_irq()
                                                         netif_tx_wake_queue()
   ena_napi_disable_all()
   ena_destroy_all_io_queues()

After these flows the tx queue is active and ena_start_xmit() accesses
the destroyed queue which leads to a kernel panic.

fixes: 1738cd3e (net: ena: Add a driver for Amazon Elastic Network Adapters (ENA))
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/amazon/ena/ena_netdev.c

@@ -823,7 +823,8 @@ static int ena_clean_tx_irq(struct ena_ring *tx_ring, u32 budget)
 		above_thresh =
 			ena_com_sq_have_enough_space(tx_ring->ena_com_io_sq,
 						     ENA_TX_WAKEUP_THRESH);
-		if (netif_tx_queue_stopped(txq) && above_thresh) {
+		if (netif_tx_queue_stopped(txq) && above_thresh &&
+		    test_bit(ENA_FLAG_DEV_UP, &tx_ring->adapter->flags)) {
 			netif_tx_wake_queue(txq);
 			u64_stats_update_begin(&tx_ring->syncp);
 			tx_ring->tx_stats.queue_wakeup++;