tg3: Fix the TX ring stall

The TX ring maintained by the tg3 driver can end up in the state, when it
has packets queued for sending but the NIC hardware is not informed, so no
progress is made. This leads to a multi-second interruption in network
traffic followed by dev_watchdog() firing and resetting the queue.

The specific sequence of steps is:

1. tg3_start_xmit() is called at least once and queues packet(s) without
   updating tnapi->prodmbox (netdev_xmit_more() returns true)
2. tg3_start_xmit() is called with an SKB which causes tg3_tso_bug() to be
   called.
3. tg3_tso_bug() determines that the SKB is too large, ...

        if (unlikely(tg3_tx_avail(tnapi) <= frag_cnt_est)) {

   ... stops the queue, and returns NETDEV_TX_BUSY:

        netif_tx_stop_queue(txq);
        ...
        if (tg3_tx_avail(tnapi) <= frag_cnt_est)
                return NETDEV_TX_BUSY;

4. Since all tg3_tso_bug() call sites directly return, the code updating
   tnapi->prodmbox is skipped.

5. The queue is stuck now. tg3_start_xmit() is not called while the queue
   is stopped. The NIC is not processing new packets because
   tnapi->prodmbox wasn't updated. tg3_tx() is not called by
   tg3_poll_work() because the all TX descriptions that could be freed has
   been freed:

        /* run TX completion thread */
        if (tnapi->hw_status->idx[0].tx_consumer != tnapi->tx_cons) {
                tg3_tx(tnapi);

6. Eventually, dev_watchdog() fires triggering a reset of the queue.

This fix makes sure that the tnapi->prodmbox update happens regardless of
the reason tg3_start_xmit() returned.

Signed-off-by: Alex Pakhunov <alexey.pakhunov@spacex.com>
Signed-off-by: Vincent Wong <vincent.wong2@spacex.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>