avoid reusing cached []byte

this logic is just to ensure 100% correct.

fuse message, data+message hearder > 512 will not be cached.

weed/filesys/filehandle.go

@@ -143,6 +143,11 @@ func (fh *FileHandle) Write(ctx context.Context, req *fuse.WriteRequest, resp *f
 
 	// write the request to volume servers
 	data := req.Data
+	if len(data) <= 512 {
+		// fuse message cacheable size
+		data = make([]byte, len(req.Data))
+		copy(data, req.Data)
+	}
 
 	fh.f.entry.Attributes.FileSize = uint64(max(req.Offset+int64(len(data)), int64(fh.f.entry.Attributes.FileSize)))
 	glog.V(4).Infof("%v write [%d,%d) %d", fh.f.fullpath(), req.Offset, req.Offset+int64(len(req.Data)), len(req.Data))