Revert "block: cgroups, kconfig, build bits for BFQ-v6-3.4"

This reverts commit aeb79a8d8ba8ea036141cfb5b167f60c987fbb2f.

Conflicts:
	block/Kconfig.iosched
	block/Makefile
Revert "block/bfq-iosched.c: GCC 4.8.x compatibility fixup"

This reverts commit f5c1176c1c071f6ddc7357b9527ad0da50252ae4.
Revert "block: introduce the BFQ-v6 I/O sched for 3.4"

This reverts commit 4e6c7796bb3928b46f0647640b22a7bd9ffcf80d.
block: introduce the BFQ v7r4 io sched
block: bfq: add early queue merge to BFQ v7r4
block: cgroups and kconfig build bits for BFQ v7r4

Conflicts:
	block/Kconfig.iosched
	block/Makefile
block: update bfq to v7r5
block: Switch from BFQ-v7r5 for 3.4.0 to BFQ-v7r6 for 3.4.0

. IMPROVEMENT: Introduced a new mechanism that helps get the job done
  more quickly for services and applications that create or reactivate
  many parallel I/O-bound processes. Examples are systemd at boot, or
  commands such as git grep.
. CODE IMPROVEMENTS: Small code cleanups and improvements.

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