mount: set file write time correct to work well with rsync

fix https://github.com/chrislusf/seaweedfs/issues/2867

weed/mount/weedfs_attr.go

@@ -98,15 +98,14 @@ func (wfs *WFS) SetAttr(cancel <-chan struct{}, input *fuse.SetAttrIn, out *fuse
 		}
 	}
 
-	if mtime, ok := input.GetMTime(); ok {
-		entry.Attributes.Mtime = mtime.Unix()
-	}
-
 	if atime, ok := input.GetATime(); ok {
 		entry.Attributes.Mtime = atime.Unix()
 	}
 
-	entry.Attributes.Mtime = time.Now().Unix()
+	if mtime, ok := input.GetMTime(); ok {
+		entry.Attributes.Mtime = mtime.Unix()
+	}
+
 	out.AttrValid = 1
 	wfs.setAttrByPbEntry(&out.Attr, input.NodeId, entry)