rnbd-clt: open code send_msg_open in rnbd_clt_map_device

rnbd-clt: open code send_msg_open in rnbd_clt_map_device

Let's open code it in rnbd_clt_map_device, then we can use information
from rsp to setup gendisk and request_queue in next commits. After that,
we can remove some members (wc, fua and max_hw_sectors etc) from struct
rnbd_clt_dev.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20220706133152.12058-2-guoqing.jiang@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>