block: Switch from BFQ-v5r1 for 3.1.0 to BFQ-v6 for 3.1.0.

Improvements:

- Replacement of the cooperating-queue merging mechanism borrowed from
  CFQ with Early Queue Merge (EQM), a unified mechanism to get a
  sequential read pattern, and hence a high throughput with any set of
  processes performing interleaved I/O.
  The  main idea  is checking newly arrived requests against the next
  request of the active queue both in case of actual request insert and
  in case of request merge. EQM  also preserves the typical low-latency
  properties of  BFQ, by properly restoring the weight-raising state of
  a queue when it gets back to a non-merged state (see
  http://algo.ing.unimo.it/people/paolo/disk_sched/description.php for
  more details). The code for detecting whether two queues have to be
  merged is a slightly modified version of the CFQ code for detecting
  whether two queues belong to cooperating processes and whether the
  service of a queue should be preempted to boost the throughput.
- Fix a bug that caused the peak rate of a disk to be computed as zero
  in case of multiple I/O errors. Subsequent estimations of the weight
  raising duration caused a division-by-zero error.

Signed-off-by: Mauro Andreolini <mauro.andreolini@unimore.it>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Reviewed-by: Paolo Valente <paolo.valente@unimore.it>