[SPARK-17710][HOTFIX] Fix ClassCircularityError in ReplSuite tests in Maven build: use 'Class.forName' instead of 'Utils.classForName'

## What changes were proposed in this pull request?
Fix ClassCircularityError in ReplSuite tests when Spark is built by Maven build.

## How was this patch tested?
(1)
```
build/mvn -DskipTests -Phadoop-2.3 -Pyarn -Phive -Phive-thriftserver -Pkinesis-asl -Pmesos clean package
```
Then test:
```
build/mvn -Dtest=none -DwildcardSuites=org.apache.spark.repl.ReplSuite test
```
ReplSuite tests passed

(2)
Manual Tests against some Spark applications in Yarn client mode and Yarn cluster mode. Need to check if spark caller contexts are written into HDFS hdfs-audit.log and Yarn RM audit log successfully.

Author: Weiqing Yang <yangweiqing001@gmail.com>

Closes #15286 from Sherry302/SPARK-16757.

Author: weiqingy

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

@@ -2489,8 +2489,10 @@ private[spark] class CallerContext(
   def setCurrentContext(): Boolean = {
     var succeed = false
     try {
-      val callerContext = Utils.classForName("org.apache.hadoop.ipc.CallerContext")
-      val Builder = Utils.classForName("org.apache.hadoop.ipc.CallerContext$Builder")
+      // scalastyle:off classforname
+      val callerContext = Class.forName("org.apache.hadoop.ipc.CallerContext")
+      val Builder = Class.forName("org.apache.hadoop.ipc.CallerContext$Builder")
+      // scalastyle:on classforname
       val builderInst = Builder.getConstructor(classOf[String]).newInstance(context)
       val hdfsContext = Builder.getMethod("build").invoke(builderInst)
       callerContext.getMethod("setCurrent", callerContext).invoke(null, hdfsContext)