If NBD_DISCONNECT_ON_CLOSE is set on a device, then the driver will
issue a disconnect from nbd_release if the device has no remaining
bdev->bd_openers.

Fix ret val so reconfigure with only setting the flag succeeds.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NDoron Roberts-Kedes <doronrk@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>