blk-mq: collapse __blk_mq_drain_queue() into blk_mq_freeze_queue()

Keeping __blk_mq_drain_queue() as a separate function doesn't buy us
anything and it's gonna be further simplified.  Let's flatten it into
its caller.

This patch doesn't make any functional change.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -108,8 +108,16 @@ static void blk_mq_queue_exit(struct request_queue *q)
 	__percpu_counter_add(&q->mq_usage_counter, -1, 1000000);
 }
 
-void blk_mq_drain_queue(struct request_queue *q)
+/*
+ * Guarantee no request is in use, so we can change any data structure of
+ * the queue afterward.
+ */
+void blk_mq_freeze_queue(struct request_queue *q)
 {
+	spin_lock_irq(q->queue_lock);
+	q->mq_freeze_depth++;
+	spin_unlock_irq(q->queue_lock);
+
 	while (true) {
 		s64 count;
 
@@ -124,19 +132,6 @@ void blk_mq_drain_queue(struct request_queue *q)
 	}
 }
 
-/*
- * Guarantee no request is in use, so we can change any data structure of
- * the queue afterward.
- */
-void blk_mq_freeze_queue(struct request_queue *q)
-{
-	spin_lock_irq(q->queue_lock);
-	q->mq_freeze_depth++;
-	spin_unlock_irq(q->queue_lock);
-
-	blk_mq_drain_queue(q);
-}
-
 static void blk_mq_unfreeze_queue(struct request_queue *q)
 {
 	bool wake = false;