Set parallelism for the parallelize job in recursiveListDirs (delta-i…

…o#3708)

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#### Which Delta project/connector is this regarding?
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- [x] Spark
- [ ] Standalone
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## Description

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`DeltaFileOperations.recursiveListDirs` calls `parallelize` without
specifying the parallelism. Hence, it always uses [the number of
available cores on a
cluster](https://github.com/apache/spark/blob/d2e8c1cb60e34a1c7e92374c07d682aa5ca79145/core/src/main/scala/org/apache/spark/SparkContext.scala#L1003).
When a cluster has many cores but `subDirs` is small, it will launch
many empty tasks.

This PR makes a small change to use
`subDirs.length.min(spark.sparkContext.defaultParallelism)` as the
parallelism so that when `subDirs` is smaller than the number of
available cores, it will not launch empty tasks.

## How was this patch tested?

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Existing tests.

## Does this PR introduce _any_ user-facing changes?

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No

spark/src/main/scala/org/apache/spark/sql/delta/util/DeltaFileOperations.scala

@@ -243,7 +243,10 @@ object DeltaFileOperations extends DeltaLogging {
     import org.apache.spark.sql.delta.implicits._
     if (subDirs.isEmpty) return spark.emptyDataset[SerializableFileStatus]
     val listParallelism = fileListingParallelism.getOrElse(spark.sparkContext.defaultParallelism)
-    val dirsAndFiles = spark.sparkContext.parallelize(subDirs).mapPartitions { dirs =>
+    val subDirsParallelism = subDirs.length.min(spark.sparkContext.defaultParallelism)
+    val dirsAndFiles = spark.sparkContext.parallelize(
+        subDirs,
+        subDirsParallelism).mapPartitions { dirs =>
       val logStore = LogStore(SparkEnv.get.conf, hadoopConf.value.value)
       listUsingLogStore(
         logStore,