blk: make blk-rq-qos policies pluggable and modular

Hi Jens

blk-rq-qos is a standalone framework out of io-sched and can be used to
control or observe the IO progress in block-layer with hooks. blk-rq-qos
is a great design but right now, it is totally fixed and built-in and shut
out peoples who want to use it with external module.

This patchset attempts to make blk-rq-qos framework pluggable and modular.
Then we can update the blk-rq-qos policy module w/o stopping the IO workload.
And it is more convenient to introduce new policy on old machines w/o udgrade
kernel. And we can close all of the blk-rq-qos policy if we needn't any of
them. At the moment, the request_queue.rqos list is empty, we needn't to
waste cpu cyles on them.

Changes since v2:
 Refactor the patches,
 - patch01 only provide new interfaces and not export the sysfs interface.
 - patch02 adapt the wbt to new interface, reserve the previous action
   when write to wbt_lat
 - export sysfs interface in last patch when all of the blk-rqos policies
   are preprared well.

Changes since v1:
 - Just make iocost and iolatency pluggable, then we needn't to export
   those interfaces
 - Remove the iostat rqos policy
 - Rename module of blk-ioprio to io-prio to avoid rename ioprio.c file

WangJianchao(6):
	blk: prepare to make blk-rq-qos pluggable and modular
    blk-wbt: make wbt pluggable
    blk-iolatency: make iolatency pluggable
    blk-iocost: make iocost pluggable
    blk-ioprio: make ioprio pluggable and modular
    blk: export the sysfs for switching qos

 block/Kconfig          |   2 +-
 block/Makefile         |   3 +-
 block/blk-cgroup.c     |  11 ---
 block/blk-iocost.c     |  49 +++++++-----
 block/blk-iolatency.c  |  33 +++++++--
 block/blk-ioprio.c     |  50 ++++++++-----
 block/blk-ioprio.h     |  19 -----
 block/blk-mq-debugfs.c |  23 ++----
 block/blk-rq-qos.c     | 303 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 block/blk-rq-qos.h     |  96 +++++++++---------------
 block/blk-sysfs.c      |   2 +
 block/blk-wbt.c        |  52 +++++++++++--
 block/blk-wbt.h        |   7 +-
 block/blk.h            |   6 --
 include/linux/blkdev.h |   4 +
 15 files changed, 488 insertions(+), 172 deletions(-)