blk-throttle: use 'READ/WRITE' instead of '0/1'

Make the code easier to read, like everywhere else.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220827101637.1775111-2-yukuai1@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>