block/mq-deadline: Introduce dd_start_request()

block/mq-deadline: Introduce dd_start_request()

[ Upstream commit 93a358af59c6e8ab00b57cfdb1c437516a4948ca ]

Prepare for adding a second caller of this function. No functionality
has been changed.

Cc: Damien Le Moal <dlemoal@kernel.org>
Cc: Yu Kuai <yukuai@kernel.org>
Cc: chengkaitao <chengkaitao@kylinos.cn>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: d60055cf5270 ("block/mq-deadline: Switch back to a single dispatch list")
Signed-off-by: Sasha Levin <sashal@kernel.org>