blk-mq: introduce a blk_mq_peek_cached_request helper

Add a new helper to check if there is suitable cached request in
blk_mq_submit_bio.  This removes open coded logic in blk_mq_submit_bio
and moves some checks that so far are in blk_mq_use_cached_rq to
be performed earlier.  This avoids the case where we first do check
with the cached request but then later end up allocating a new one
anyway and need to grab a queue reference.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240124092658.2258309-3-hch@lst.de


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