blk-stat: export helpers for modifying blk_rq_stat

We need to use blk_rq_stat in the blkcg qos stuff, so export some of
these helpers so they can be used by other things.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>