nbd: don't leak nbd_config

nbd_config is allocated in nbd_alloc_config(), but never freed.

Fixes: 5ea8d10802ec ("nbd: separate out the config information")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/nbd.c

@@ -1021,6 +1021,7 @@ static void nbd_config_put(struct nbd_device *nbd)
 			}
 			kfree(config->socks);
 		}
+		kfree(nbd->config);
 		nbd->config = NULL;
 
 		nbd->tag_set.timeout = 0;