blk-mq: special case cached requests less

Share the main merge / split / integrity preparation code between the
cached request vs newly allocated request cases, and add comments
explaining the cached request handling.

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/20240124092658.2258309-4-hch@lst.de


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