cfq-iosched: implement slower async initiate and queue ramp up

This slowly ramps up the async queue depth based on the time
passed since the sync IO, and doesn't allow async at all until
a sync slice period has passed.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/blkdev.h

@@ -1147,7 +1147,11 @@ static inline void put_dev_sector(Sector p)
 }
 
 struct work_struct;
+struct delayed_work;
 int kblockd_schedule_work(struct request_queue *q, struct work_struct *work);
+int kblockd_schedule_delayed_work(struct request_queue *q,
+					struct delayed_work *work,
+				 	unsigned long delay);
 
 #define MODULE_ALIAS_BLOCKDEV(major,minor) \
 	MODULE_ALIAS("block-major-" __stringify(major) "-" __stringify(minor))