Do not return error when rootfs already exists.

Signed-off-by: Lantao Liu <lantaol@google.com>

runtime/v1/shim/service.go

@@ -127,7 +127,7 @@ func (s *Service) Create(ctx context.Context, r *shimapi.CreateTaskRequest) (_ *
 	rootfs := ""
 	if len(mounts) > 0 {
 		rootfs = filepath.Join(r.Bundle, "rootfs")
-		if err := os.Mkdir(rootfs, 0711); err != nil {
+		if err := os.Mkdir(rootfs, 0711); err != nil && !os.IsExist(err) {
 			return nil, err
 		}
 	}