Revert "block: inherit request start time from bio for BLK_CGROUP"

This reverts commit 0006707723233cb2a9a23ca19fc3d0864835704c. 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 <[email protected]>
Cc: [email protected] # v5.16+
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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