rnbd-srv: simplify rnbd_srv_fill_msg_open_rsp

Remove all the wrappers and just get the information directly from
the block device, or where no such helpers exist the request_queue.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20220909131509.3263924-2-hch@lst.de


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