null_blk: Modify the behavior of null_map_queues()

Instead of returning -EINVAL if an internal inconsistency is detected,
fall back to a single submission queue. This patch prepares for changing
the return value of the .map_queues() callbacks into void.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220815170043.19489-2-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -1555,7 +1555,9 @@ static int null_map_queues(struct blk_mq_tag_set *set)
 		} else {
 			pr_warn("tag set has unexpected nr_hw_queues: %d\n",
 				set->nr_hw_queues);
-			return -EINVAL;
+			WARN_ON_ONCE(true);
+			submit_queues = 1;
+			poll_queues = 0;
 		}
 	}