SPARK-3190 was originally fixed by 96df9290, but a5ef5811 introduced a regression during refactoring. This commit fixes the regression.

Author: Ankur Dave <ankurdave@gmail.com>

Closes #7923 from ankurdave/SPARK-3190-reopening and squashes the following commits:

a3e1b23 [Ankur Dave] Fix VertexRDD.count() overflow regression