blk-rq-qos: fix first node deletion of rq_qos_del()

rq_qos_del() incorrectly assigns the node being deleted to the head if
it was the first on the list in the !prev path.  Fix it by iterating
with ** instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Fixes: a790504 ("blk-rq-qos: refactor out common elements of blk-wbt")
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-rq-qos.h

@@ -108,16 +108,13 @@ static inline void rq_qos_add(struct request_queue *q, struct rq_qos *rqos)
 
 static inline void rq_qos_del(struct request_queue *q, struct rq_qos *rqos)
 {
-	struct rq_qos *cur, *prev = NULL;
-	for (cur = q->rq_qos; cur; cur = cur->next) {
-		if (cur == rqos) {
-			if (prev)
-				prev->next = rqos->next;
-			else
-				q->rq_qos = cur;
+	struct rq_qos **cur;
+
+	for (cur = &q->rq_qos; *cur; cur = &(*cur)->next) {
+		if (*cur == rqos) {
+			*cur = rqos->next;
 			break;
 		}
-		prev = cur;
 	}
 
 	blk_mq_debugfs_unregister_rqos(rqos);