blk-mq: update nr_requests when switching to 'none' scheduler

Now we setup q->nr_requests when switching to one new scheduler,
but not do it for 'none', then q->nr_requests may not be correct
for 'none'.

This patch fixes this issue by always updating 'nr_requests' when
switching to 'none'.

Cc: Marco Patalano <mpatalan@redhat.com>
Cc: "Ewan D. Milne" <emilne@redhat.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-mq-sched.c

@@ -585,6 +585,7 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
 
 	if (!e) {
 		q->elevator = NULL;
+		q->nr_requests = q->tag_set->queue_depth;
 		return 0;
 	}