HADOOP-13527. Add Spark to CallerContext LimitedPrivate scope. (Contributed by Weiqing Yang)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/CallerContext.java

@@ -33,7 +33,7 @@ import java.util.Arrays;
  * This class is immutable.
  */
 @InterfaceAudience.LimitedPrivate({"HBase", "HDFS", "Hive", "MapReduce",
-    "Pig", "YARN"})
+    "Pig", "Spark", "YARN"})
 @InterfaceStability.Evolving
 public final class CallerContext {
   public static final Charset SIGNATURE_ENCODING = StandardCharsets.UTF_8;