[SPARK-20788][CORE] Fix the Executor task reaper's false alarm warning logs

## What changes were proposed in this pull request?

Executor task reaper may fail to detect if a task is finished or not when a task is finishing but being killed at the same time.

The fix is pretty easy, just flip the "finished" flag when a task is successful.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18021 from zsxwing/SPARK-20788.

core/src/main/scala/org/apache/spark/executor/Executor.scala

@@ -425,6 +425,7 @@ private[spark] class Executor(
           }
         }
 
+        setTaskFinishedAndClearInterruptStatus()
         execBackend.statusUpdate(taskId, TaskState.FINISHED, serializedResult)
 
       } catch {