[Spark Core] SPARK-4380 Edit spilling log from MB to B

https://issues.apache.org/jira/browse/SPARK-4380

Author: Hong Shen <hongshen@tencent.com>

Closes #3243 from shenh062326/spark_change and squashes the following commits:

4653378 [Hong Shen] Edit spilling log from MB to B
21ee960 [Hong Shen] Edit spilling log from MB to B
e9145e8 [Hong Shen] Edit spilling log from MB to B
da761c2 [Hong Shen] Edit spilling log from MB to B
946351c [Hong Shen] Edit spilling log from MB to B

core/src/main/scala/org/apache/spark/util/collection/Spillable.scala

@@ -105,7 +105,8 @@ private[spark] trait Spillable[C] {
    */
   @inline private def logSpillage(size: Long) {
     val threadId = Thread.currentThread().getId
-    logInfo("Thread %d spilling in-memory map of %d MB to disk (%d time%s so far)"
-        .format(threadId, size / (1024 * 1024), _spillCount, if (_spillCount > 1) "s" else ""))
+    logInfo("Thread %d spilling in-memory map of %s to disk (%d time%s so far)"
+        .format(threadId, org.apache.spark.util.Utils.bytesToString(size),
+            _spillCount, if (_spillCount > 1) "s" else ""))
   }
 }