block: Switch from BFQ-v7r4 for 3.15.0 to BFQ-v7r5 for 3.15.0

. IMPROVEMENT: Improve throughput boosting by idling the device
  only for processes that, in addition to perform sequential I/O,
  are I/O-bound (apart from weight-raised queues, for which idling
  is always performed to guarantee them a low latency).
. IMPROVEMENT: Improve throughput boosting by depriving processes
  that cooperate often of weight-raising.
. CODE IMPROVEMENT: Pass of improvement of the readability of both
  comments and actual code.

Signed-off-by: Paolo Valente <paolo.valente@unimore.it>
Tested-by: Takashi Iwai <tiwai@suse.de>
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Merge remote-tracking branch 'github/3.15/bfq' into 3.15/master