Fix stress test tool exec kill

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/containerd-stress/main.go

@@ -239,10 +239,10 @@ func (w *worker) runContainer(ctx context.Context, id string) error {
 				logrus.WithError(err).Error("exec failure")
 			}
 		}
-	}
 		if err := task.Kill(ctx, syscall.SIGKILL); err != nil {
 			return err
 		}
+	}
 	status := <-statusC
 	_, _, err = status.Result()
 	if err != nil {