HADOOP-18944. S3A: openfile to set async drain threshold correctly

Fix default value of Async drain threshold in S3A OpenFile

Contributed by Mehakmeet Singh

Author: mehakmeet

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/impl/OpenFileSupport.java

@@ -286,7 +286,7 @@ public OpenFileInformation prepareToOpenFile(
     return new OpenFileInformation()
         .withAsyncDrainThreshold(
             builderSupport.getPositiveLong(ASYNC_DRAIN_THRESHOLD,
-                defaultReadAhead))
+                defaultAsyncDrainThreshold))
         .withBufferSize(
             (int)builderSupport.getPositiveLong(
                 FS_OPTION_OPENFILE_BUFFER_SIZE, defaultBufferSize))