[FIX JENKINS-25449] Form validation for label expression

[JENKINS-25449](https://issues.jenkins-ci.org/browse/JENKINS-25449)
![matrix-expr-validate](https://cloud.githubusercontent.com/assets/206841/4919049/c796556a-64f6-11e4-9d3d-d07be66548db.png)

FYI this is moving to @Restricted(DoNoUse) in recent cores because it has been renamed.  If you update the core the new name is doCheckLabel.