block: Update to BFQv7r6

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.

Signed-off-by: Chet Kener <Cl3Kener@gmail.com>