[SPARK-3190] [GRAPHX] Fix VertexRDD.count() overflow regression

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

graphx/src/main/scala/org/apache/spark/graphx/impl/VertexRDDImpl.scala

@@ -87,7 +87,7 @@ class VertexRDDImpl[VD] private[graphx] (
 
   /** The number of vertices in the RDD. */
   override def count(): Long = {
-    partitionsRDD.map(_.size).reduce(_ + _)
+    partitionsRDD.map(_.size.toLong).reduce(_ + _)
   }
 
   override private[graphx] def mapVertexPartitions[VD2: ClassTag](