block: fix elv_update_nr_hw_queues() to reattach elevator

When nr_hw_queues is updated, the elevator needs to be switched to
ensure that we exit elevator and reattach it to ensure that hctx->
sched_tags is correctly allocated for the new hardware queues.
However, elv_update_nr_hw_queues() currently only switches the
elevator if the queue is not registered. This is incorrect, as it
prevents reattaching the elevator after updating nr_hw_queues, which
in turn inhibits allocation of sched_tags.

Fix this by allowing the elevator switch if the queue is registered,
ensuring proper reattachment and resource allocation.

Fixes: 596dce1 ("block: simplify elevator reattachment for updating nr_hw_queues")
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250515134511.548270-1-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/elevator.c

@@ -697,7 +697,7 @@ void elv_update_nr_hw_queues(struct request_queue *q)
 	WARN_ON_ONCE(q->mq_freeze_depth == 0);
 
 	mutex_lock(&q->elevator_lock);
-	if (q->elevator && !blk_queue_dying(q) && !blk_queue_registered(q)) {
+	if (q->elevator && !blk_queue_dying(q) && blk_queue_registered(q)) {
 		ctx.name = q->elevator->type->elevator_name;
 
 		/* force to reattach elevator after nr_hw_queue is updated */