block, bfq: factor out a helper to split bfqq in bfq_init_rq()

Make code cleaner, there are no functional changes.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240909134154.954924-8-yukuai1@huaweicloud.com


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