Fixed fdatasync build errors on linux.

fsutil/fdatasync_linux.go

+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fsutil
+
+import (
+	"os"
+	"syscall"
+)
+
+const FdatasyncSupported = true
+
+func fdatasync(f *os.File) error {
+	return syscall.Fdatasync(int(f.Fd()))
+}