block, bfq: do not update peak rate if no I/O was done during the service round

It may happen that, due to I/O errors, the active queue is expired
without any of its requests being actually completed. If this happens,
do not update the peak rate.
block, bfq: do not update peak rate if service rate of the active queue is zero

It may happen that the amount of service received by the active
queue is so small that the computed service rate is zero. In this
case, do not update the peak rate.
Merge branch 'bfq-3.7' into pf-3.7