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 <osandov@fb.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
                        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);
                }