vfs: convert vfs options to new style

This also
- move in use options (Opt) from vfsflags to vfscommon
- change os.FileMode to vfscommon.FileMode in parameters
- rework vfscommon.FileMode and add tests

vfs/vfscache/writeback/writeback_test.go

@@ -17,7 +17,7 @@ import (
 
 func newTestWriteBack(t *testing.T) (wb *WriteBack, cancel func()) {
 	ctx, cancel := context.WithCancel(context.Background())
-	opt := vfscommon.DefaultOpt
+	opt := vfscommon.Opt
 	opt.WriteBack = fs.Duration(100 * time.Millisecond)
 	wb = New(ctx, &opt)
 	return wb, cancel