blk-iocost: don't ignore vrate_min on QD contention

blk-iocost: don't ignore vrate_min on QD contention

ioc_adjust_base_vrate() ignored vrate_min when rq_wait_pct indicates that
there is QD contention. The reasoning was that QD depletion always reliably
indicates device saturation and thus it's safe to override user specified
vrate_min. However, this sometimes leads to unnecessary throttling,
especially on really fast devices, because vrate adjustments have delays and
inertia. It also confuses users because the behavior violates the explicitly
specified configuration.

This patch drops the special case handling so that vrate_min is always
applied.

Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/YIIo1HuyNmhDeiNx@slm.duckdns.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>