[SPARK-24307][CORE] Add conf to revert to old code.

In case there are any issues in converting FileSegmentManagedBuffer to
ChunkedByteBuffer, add a conf to go back to old code path.

Followup to 7e84764

Author: Imran Rashid <[email protected]>

Closes apache#21867 from squito/SPARK-24307-p2.

Author: squito

core/src/main/scala/org/apache/spark/storage/BlockManager.scala

@@ -132,6 +132,8 @@ private[spark] class BlockManager(
     conf.getBoolean("spark.shuffle.service.enabled", false)
   private val chunkSize =
     conf.getSizeAsBytes("spark.storage.memoryMapLimitForTests", Int.MaxValue.toString).toInt
+  private val remoteReadNioBufferConversion =
+    conf.getBoolean("spark.network.remoteReadNioBufferConversion", false)
 
   val diskBlockManager = {
     // Only perform cleanup if an external service is not serving our shuffle files.
@@ -731,7 +733,14 @@ private[spark] class BlockManager(
       }
 
       if (data != null) {
-        return Some(ChunkedByteBuffer.fromManagedBuffer(data, chunkSize))
+        // SPARK-24307 undocumented "escape-hatch" in case there are any issues in converting to
+        // ChunkedByteBuffer, to go back to old code-path.  Can be removed post Spark 2.4 if
+        // new path is stable.
+        if (remoteReadNioBufferConversion) {
+          return Some(new ChunkedByteBuffer(data.nioByteBuffer()))
+        } else {
+          return Some(ChunkedByteBuffer.fromManagedBuffer(data, chunkSize))
+        }
       }
       logDebug(s"The value of block $blockId is null")
     }