rnbd: remove rnbd_dev_submit_io

The function only has one caller, so let's open code it in process_rdma.
Another bonus is we can avoid push/pop stack, since we need to pass 8
arguments to rnbd_dev_submit_io.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Acked-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>