rnbd-srv: remove struct rnbd_dev_blk_io

Only the priv field of rnbd_dev_blk_io is used, so store the value of
that in bio->bi_private directly and remove the entire bio_set overhead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20220124091107.642561-12-hch@lst.de


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