blk-mq: factor out a blk_mq_get_budget_and_tag helper

Factor out a helper from __blk_mq_try_issue_directly in preparation
of folding that function into its two callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230413064057.707578-13-hch@lst.de


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