block: Switch from BFQ-v7r5 for 3.4.0 to BFQ-v7r6 for 3.4.0

CHANGELOG

v7r6:

. IMPROVEMENT: Introduced a new mechanism that helps get the job done
  more quickly with services and applications that create or reactivate
  many parallel I/O-bound processes. This is the case, for example, with
  systemd at boot, or with commands like git grep.

. CODE IMPROVEMENTS: Small code cleanups and improvements.