blk-rq-qos: refactor out common elements of blk-wbt

blkcg-qos is going to do essentially what wbt does, only on a cgroup
basis.  Break out the common code that will be shared between blkcg-qos
and wbt into blk-rq-qos.* so they can both utilize the same
infrastructure.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>