[SPARK-18843][CORE] Fix timeout in awaitResultInForkJoinSafely (branch 2.1, 2.0)

## What changes were proposed in this pull request?

This PR fixes the timeout value in `awaitResultInForkJoinSafely` for 2.1 and 2.0. Master has been fixed by https://github.com/apache/spark/pull/16230.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16268 from zsxwing/SPARK-18843.

core/src/main/scala/org/apache/spark/util/ThreadUtils.scala

@@ -209,7 +209,7 @@ private[spark] object ThreadUtils {
       // `awaitPermission` is not actually used anywhere so it's safe to pass in null here.
       // See SPARK-13747.
       val awaitPermission = null.asInstanceOf[scala.concurrent.CanAwait]
-      awaitable.result(Duration.Inf)(awaitPermission)
+      awaitable.result(atMost)(awaitPermission)
     } catch {
       case NonFatal(t) =>
         throw new SparkException("Exception thrown in awaitResult: ", t)