nbd: fix return value in error handling path

It seems that the proper value to return in this particular case is the
one contained into variable new_index instead of ret.

Addresses-Coverity-ID: 1465148 ("Copy-paste error")
Fixes: e46c7287b1c2 ("nbd: add a basic netlink interface")
Reviewed-by: Omar Sandoval <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/nbd.c

···
 			if (new_index < 0) {
 				mutex_unlock(&nbd_index_mutex);
 				printk(KERN_ERR "nbd: failed to add new device\n");
-				return ret;
+				return new_index;
 			}
 			nbd = idr_find(&nbd_index_idr, new_index);
 		}