null_blk: mem garbage on NUMA systems during init

For NUMA systems, initializing the blk-mq layer and using per node hctx.
We initialize submit queues to 1, while blk-mq nr_hw_queues is
initialized to the number of NUMA nodes.

This makes the null_init_hctx function overwrite memory outside of what
it allocated.  In my case it lead to writing garbage into struct
request_queue's mq_map.

Signed-off-by: Matias Bjorling <m@bjorling.me>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/null_blk.c

@@ -495,23 +495,23 @@ static int null_add_dev(void)
 
 	spin_lock_init(&nullb->lock);
 
+	if (queue_mode == NULL_Q_MQ && use_per_node_hctx)
+		submit_queues = nr_online_nodes;
+
 	if (setup_queues(nullb))
 		goto err;
 
 	if (queue_mode == NULL_Q_MQ) {
 		null_mq_reg.numa_node = home_node;
 		null_mq_reg.queue_depth = hw_queue_depth;
+		null_mq_reg.nr_hw_queues = submit_queues;
 
 		if (use_per_node_hctx) {
 			null_mq_reg.ops->alloc_hctx = null_alloc_hctx;
 			null_mq_reg.ops->free_hctx = null_free_hctx;
-
-			null_mq_reg.nr_hw_queues = nr_online_nodes;
 		} else {
 			null_mq_reg.ops->alloc_hctx = blk_mq_alloc_single_hw_queue;
 			null_mq_reg.ops->free_hctx = blk_mq_free_single_hw_queue;
-
-			null_mq_reg.nr_hw_queues = submit_queues;
 		}
 
 		nullb->q = blk_mq_init_queue(&null_mq_reg, nullb);