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.
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