Refactor for Sync method (#3426)

weed/storage/backend/disk_file_linux.go

@@ -0,0 +1,14 @@
+//go:build linux
+// +build linux
+
+package backend
+
+import (
+	"syscall"
+)
+
+// Using Fdatasync to optimize file sync operation
+func (df *DiskFile) Sync() error {
+	fd := df.File.Fd()
+	return syscall.Fdatasync(int(fd))
+}