blk-mq: move more plug handling from blk_mq_submit_bio into blk_add_rq_to_plug

Keep all the functionality for adding a request to a plug in a single place.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211123160443.1315598-3-hch@lst.de


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