block: take rq_list instead of plug in dispatch functions

blk_mq_plug_issue_direct(), __blk_mq_flush_plug_list(), and
blk_mq_dispatch_plug_list() take a struct blk_plug * but only use its
mq_list. Pass the struct rq_list * instead in preparation for calling
them with other lists of requests.

Drop "plug" from the function names as they are no longer plug-specific.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250426011728.4189119-2-csander@purestorage.com


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