[SPARK-22465][CORE][FOLLOWUP] Use existing partitioner when defaultNumPartitions is equal to maxPartitioner.numPartitions

## What changes were proposed in this pull request?

Followup of apache#20091. We could also use existing partitioner when defaultNumPartitions is equal to the maxPartitioner's numPartitions.

## How was this patch tested?

Existed.

Closes apache#23581 from Ngone51/dev-use-existing-partitioner-when-defaultNumPartitions-equalTo-MaxPartitioner#-numPartitions.

Authored-by: Ngone51 <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: Ngone51

core/src/main/scala/org/apache/spark/Partitioner.scala

@@ -51,8 +51,8 @@ object Partitioner {
    *
    * When available, we choose the partitioner from rdds with maximum number of partitions. If this
    * partitioner is eligible (number of partitions within an order of maximum number of partitions
-   * in rdds), or has partition number higher than default partitions number - we use this
-   * partitioner.
+   * in rdds), or has partition number higher than or equal to default partitions number - we use
+   * this partitioner.
    *
    * Otherwise, we'll use a new HashPartitioner with the default partitions number.
    *
@@ -79,9 +79,9 @@ object Partitioner {
     }
 
     // If the existing max partitioner is an eligible one, or its partitions number is larger
-    // than the default number of partitions, use the existing partitioner.
+    // than or equal to the default number of partitions, use the existing partitioner.
     if (hasMaxPartitioner.nonEmpty && (isEligiblePartitioner(hasMaxPartitioner.get, rdds) ||
-        defaultNumPartitions < hasMaxPartitioner.get.getNumPartitions)) {
+        defaultNumPartitions <= hasMaxPartitioner.get.getNumPartitions)) {
       hasMaxPartitioner.get.partitioner.get
     } else {
       new HashPartitioner(defaultNumPartitions)