cfq-iosched: reduce write depth only if sync was delayed

The introduction of ramp-up formula for async queue depths has
slowed down dirty page reclaim, by reducing async write performance.
This patch makes sure the formula kicks in only when sync request
was recently delayed.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>