[SPARK-12678][CORE] MapPartitionsRDD clearDependencies

MapPartitionsRDD was keeping a reference to `prev` after a call to
`clearDependencies` which could lead to memory leak.

Author: Guillaume Poulin <poulin.guillaume@gmail.com>

Closes #10623 from gpoulin/map_partition_deps.

core/src/main/scala/org/apache/spark/rdd/MapPartitionsRDD.scala

@@ -25,7 +25,7 @@ import org.apache.spark.{Partition, TaskContext}
  * An RDD that applies the provided function to every partition of the parent RDD.
  */
 private[spark] class MapPartitionsRDD[U: ClassTag, T: ClassTag](
-    prev: RDD[T],
+    var prev: RDD[T],
     f: (TaskContext, Int, Iterator[T]) => Iterator[U],  // (TaskContext, partition index, iterator)
     preservesPartitioning: Boolean = false)
   extends RDD[U](prev) {
@@ -36,4 +36,9 @@ private[spark] class MapPartitionsRDD[U: ClassTag, T: ClassTag](
 
   override def compute(split: Partition, context: TaskContext): Iterator[U] =
     f(context, split.index, firstParent[T].iterator(split, context))
+
+  override def clearDependencies() {
+    super.clearDependencies()
+    prev = null
+  }
 }