block: avoid setting none scheduler if it's already none

BugLink: https://bugs.launchpad.net/bugs/1815733

There's no reason to freeze queue and remove scheduler
if there's no scheduler already.

Signed-off-by: Aleksei Zakharov <zakharov.a.g@yandex.ru>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(backported from commit fbd7212)
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Acked-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

block/elevator.c

@@ -1090,8 +1090,11 @@ static int __elevator_change(struct request_queue *q, const char *name)
 	/*
 	 * Special case for mq, turn off scheduling
 	 */
-	if (q->mq_ops && !strncmp(name, "none", 4))
+	if (q->mq_ops && !strncmp(name, "none", 4)) {
+		if (!q->elevator)
+			return 0;
 		return elevator_switch(q, NULL);
+	}
 
 	strlcpy(elevator_name, name, sizeof(elevator_name));
 	e = elevator_get(q, strstrip(elevator_name), true);