block: consolidate struct request timestamp fields

Currently, struct request has four timestamp fields:

- A start time, set at get_request time, in jiffies, used for iostats
- An I/O start time, set at start_request time, in ktime nanoseconds,
  used for blk-stats (i.e., wbt, kyber, hybrid polling)
- Another start time and another I/O start time, used for cfq and bfq

These can all be consolidated into one start time and one I/O start
time, both in ktime nanoseconds, shaving off up to 16 bytes from struct
request depending on the kernel config.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-rq.c

@@ -406,7 +406,7 @@ static blk_status_t dm_dispatch_clone_request(struct request *clone, struct requ
 	if (blk_queue_io_stat(clone->q))
 		clone->rq_flags |= RQF_IO_STAT;
 
-	clone->start_time = jiffies;
+	clone->start_time_ns = ktime_get_ns();
 	r = blk_insert_cloned_request(clone->q, clone);
 	if (r != BLK_STS_OK && r != BLK_STS_RESOURCE && r != BLK_STS_DEV_RESOURCE)
 		/* must complete clone in terms of original request */