block: Switch from BFQ-v7r6 for 3.18.0 to BFQ-v7r7 for 3.18.0

. BUGFIX: Prevent the OOM queue from being involved in the queue
  cooperation mechanism. In fact, since the requests temporarily
  redirected to the OOM queue could be redirected again to dedicated
  queues at any time, the state needed to correctly handle merging
  with the OOM queue would be quite complex and expensive to
  maintain. Besides, in such a critical condition as an out of
  memory, the benefits of queue merging may be little relevant, or
  even negligible.

. IMPROVEMENT: Let the OOM queue be initialized only once. Previously,
  the OOM queue was reinitialized, at each request enqueue, with the
  parameters related to the process that issued that request.
  Depending on the parameters of the processes doing I/O, this could
  easily cause the OOM queue to be moved continuously across service
  trees, or even across groups. It also caused the parameters of the
  OOM queue to be continuously reset in any case.

. CODE IMPROVEMENT. Performed some minor code cleanups, and added some
  BUG_ON()s that, if the weight of an entity becomes inconsistent,
  should better help understand why.

Signed-off-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Reported-by: Hector Martin <hector@marcansoft.com>