s390/qeth: allocate a single cmd on read channel

We statically allocate 8 cmd buffers on the read channel, when the only
IO left that's still using them is the long-running READ.
Replace this with a single allocated cmd, that gets restarted whenever
the READ completed.

This introduces refcounting for allocated cmds, so that the READ cmd can
survive the IO completion.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/s390/net/qeth_l3_main.c

@@ -1436,7 +1436,6 @@ static void qeth_l3_stop_card(struct qeth_card *card)
 		card->state = CARD_STATE_DOWN;
 	}
 
-	qeth_clear_cmd_buffers(&card->read);
 	qeth_clear_cmd_buffers(&card->write);
 	flush_workqueue(card->event_wq);
 }