blk-mq: make function '__blk_mq_sched_dispatch_requests' static

Fix sparse warnings:

block/blk-mq-sched.c:209:5: warning: symbol '__blk_mq_sched_dispatch_requests' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-sched.c

@@ -206,7 +206,7 @@ static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
 	return ret;
 }
 
-int __blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
+static int __blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 {
 	struct request_queue *q = hctx->queue;
 	struct elevator_queue *e = q->elevator;