nbd: rename the runtime flags as NBD_RT_ prefixed

mainline inclusion
from mainline-5.4-rc1
commit ec76a7b9
category: bugfix
bugzilla: 23253
CVE: NA
---------------------------

Preparing for the destory when disconnecting crash fixing.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sun Ke <sunke32@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>