blk-wbt: rename __wbt_update_limits to wbt_update_limits

Now let's rename __wbt_update_limits to wbt_update_limits after the
previous one is deleted.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>