test/file-register: skip file skipping on older kernels

All new op testing should include this, so that tests continue to
work on older kernels...

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/test/file-register.c b/test/file-register.c
index 0f4d679..b86ee59 100644
--- a/test/file-register.c
+++ b/test/file-register.c
@@ -565,6 +565,10 @@
 	files[90] = IORING_REGISTER_FILES_SKIP;
 	ret = io_uring_register_files_update(ring, 90, &files[90], 1);
 	if (ret != 1) {
+		if (ret == -EBADF) {
+			fprintf(stdout, "Skipping files not supported\n");
+			goto done;
+		}
 		fprintf(stderr, "%s: update ret=%d\n", __FUNCTION__, ret);
 		goto err;
 	}
@@ -579,6 +583,7 @@
 		goto err;
 	}
 
+done:
 	close_files(files, 100, 0);
 	return 0;
 err: