blk-iocost: streamline vtime margin and timer slack handling

The margin handling was pretty inconsistent.

* ioc->margin_us and ioc->inuse_margin_vtime were used as vtime margin
  thresholds. However, the two are in different units with the former
  requiring conversion to vtime on use.

* iocg_kick_waitq() was using a quarter of WAITQ_TIMER_MARGIN_PCT of
  period_us as the timer slack - ~1.2%. While iocg_kick_delay() was using a
  quarter of ioc->margin_us - ~12.5%. There aren't strong reasons to use
  different values for the two.

This patch cleans up margin and timer slack handling:

* vtime margins are now recorded in ioc->margins.{min, max} on period
  duration changes and used consistently.

* Timer slack is now 1% of period_us and recorded in ioc->timer_slack_ns and
  used consistently for iocg_kick_waitq() and iocg_kick_delay().

The only functional change is shortening of timer slack. No meaningful
visible change is expected.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>