Use O_EXCL when creating file as we never overwrite an existing file (#7189)

cmd/xl-v1-object_test.go

@@ -323,6 +323,11 @@ func TestHealing(t *testing.T) {
 		t.Fatal("HealObject failed")
 	}
 
+	err = os.RemoveAll(path.Join(fsDirs[0], bucket, object, "xl.json"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	// Write xl.json with different modtime to simulate the case where a disk had
 	// gone down when an object was replaced by a new object.
 	xlMetaOutDated := xlMetaPreHeal