Currently, blkcg policy implementations manage blkg refcnt duplicating
mostly identical code in both policies.  This patch moves refcnt to
blkg and let blkcg core handle refcnt and freeing of blkgs.

* cfq blkgs now also get freed via RCU.

* cfq blkgs lose RB_EMPTY_ROOT() sanity check on blkg free.  If
  necessary, we can add blkio_exit_group_fn() to resurrect this.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>