blk-wbt: Declare local functions static

This patch avoids that sparse reports the following warnings:

  CHECK   block/blk-wbt.c
block/blk-wbt.c:600:6: warning: symbol 'wbt_issue' was not declared. Should it be static?
block/blk-wbt.c:620:6: warning: symbol 'wbt_requeue' was not declared. Should it be static?
  CC      block/blk-wbt.o
block/blk-wbt.c:600:6: warning: no previous prototype for wbt_issue [-Wmissing-prototypes]
 void wbt_issue(struct rq_qos *rqos, struct request *rq)
      ^~~~~~~~~
block/blk-wbt.c:620:6: warning: no previous prototype for wbt_requeue [-Wmissing-prototypes]
 void wbt_requeue(struct rq_qos *rqos, struct request *rq)
      ^~~~~~~~~~~

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-wbt.c

@@ -597,7 +597,7 @@ static void wbt_track(struct rq_qos *rqos, struct request *rq, struct bio *bio)
 	rq->wbt_flags |= bio_to_wbt_flags(rwb, bio);
 }
 
-void wbt_issue(struct rq_qos *rqos, struct request *rq)
+static void wbt_issue(struct rq_qos *rqos, struct request *rq)
 {
 	struct rq_wb *rwb = RQWB(rqos);
 
@@ -617,7 +617,7 @@ void wbt_issue(struct rq_qos *rqos, struct request *rq)
 	}
 }
 
-void wbt_requeue(struct rq_qos *rqos, struct request *rq)
+static void wbt_requeue(struct rq_qos *rqos, struct request *rq)
 {
 	struct rq_wb *rwb = RQWB(rqos);
 	if (!rwb_enabled(rwb))