blk-mq: rename blk_mq_can_use_cached_rq

blk-mq: rename blk_mq_can_use_cached_rq

blk_mq_can_use_cached_rq doesn't just check if we can use the request,
but also performs the work to actually use it.  Remove the _can in the
naming, and improve the comment describing the function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240111135705.2155518-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>