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 '3.7/bfq' into 3.7/master
Merge branch '3.7/master' of git://github.com/damentz/zen-kernel into 3.7/my