null_blk: allow zero poll queues

There isn't any reason to not allow zero poll queues from user
viewpoint.

Also sometimes we need to compare io poll between poll mode and irq
mode, so not allowing poll queues is bad.

Fixes: 15dfc662 ("null_blk: Fix handling of submit_queues and poll_queues attributes")
Cc: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20211203023935.3424042-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -340,9 +340,9 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
 		return 0;
 
 	/*
-	 * Make sure at least one queue exists for each of submit and poll.
+	 * Make sure at least one submit queue exists.
 	 */
-	if (!submit_queues || !poll_queues)
+	if (!submit_queues)
 		return -EINVAL;
 
 	/*
@@ -1917,8 +1917,6 @@ static int null_validate_conf(struct nullb_device *dev)
 
 	if (dev->poll_queues > g_poll_queues)
 		dev->poll_queues = g_poll_queues;
-	else if (dev->poll_queues == 0)
-		dev->poll_queues = 1;
 	dev->prev_poll_queues = dev->poll_queues;
 
 	dev->queue_mode = min_t(unsigned int, dev->queue_mode, NULL_Q_MQ);