blk-mq: make __blk_mq_stop_hw_queues static

Making __blk_mq_stop_hw_queues static fixes sparse warning:

  block/blk-mq.c:6: warning: symbol '__blk_mq_stop_hw_queues' was not
  declared. Should it be static?

Fixes: 2719aa21 ("blk-mq: don't use sync workqueue flushing from drivers")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -1236,7 +1236,7 @@ void blk_mq_stop_hw_queue(struct blk_mq_hw_ctx *hctx)
 }
 EXPORT_SYMBOL(blk_mq_stop_hw_queue);
 
-void __blk_mq_stop_hw_queues(struct request_queue *q, bool sync)
+static void __blk_mq_stop_hw_queues(struct request_queue *q, bool sync)
 {
 	struct blk_mq_hw_ctx *hctx;
 	int i;