nbd: reset the setup task for NBD_CLEAR_SOCK

If an app exits before running NBD_DO_IT but after adding sockets we can
end up not being allowed to do a new nbd device.  Fix this by making
NBD_CLEAR_SOCK reset the setup_task.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/nbd.c

@@ -704,6 +704,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 			kfree(nbd->socks);
 			nbd->socks = NULL;
 			nbd->num_connections = 0;
+			nbd->task_setup = NULL;
 		}
 		return 0;