This reverts commit 00067077. It has a
couple problems:

* bio_issue_time() is stored in bio->bi_issue truncated to 51 bits. This
  overflows in slightly over 26 days. Setting rq->io_start_time_ns with it
  means that io duration calculation would yield >26days after 26 days of
  uptime. This, for example, confuses kyber making it cause high IO
  latencies.

* rq->io_start_time_ns should record the time that the IO is issued to the
  device so that on-device latency can be measured. However,
  bio_issue_time() is set before the bio goes through the rq-qos controllers
  (wbt, iolatency, iocost), so when the bio gets throttled in any of the
  mechanisms, the measured latencies make no sense - on-device latencies end
  up higher than request-alloc-to-completion latencies.

We'll need a smarter way to avoid calling ktime_get_ns() repeatedly
back-to-back. For now, let's revert the commit.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v5.16+
Link: https://lore.kernel.org/r/YmmeOLfo5lzc+8yI@slm.duckdns.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>