commit ff6b8090 upstream.

we have already allocated memory for nbd_dev, but we were not
releasing that memory and just returning the error value.
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Acked-by: Paul Clements <Paul.Clements@SteelEye.com>
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Zefan Li <lizefan@huawei.com>