Use correct system index bulk executor

In one place we would always use the Write threadpool, which could let
write thread pool queuing lead to increased system index write latency.

server/src/main/java/org/elasticsearch/action/bulk/TransportBulkAction.java

@@ -291,7 +291,7 @@ public void onTimeout(TimeValue timeout) {
     }
 
     private void forkAndExecute(Task task, BulkRequest bulkRequest, String executorName, ActionListener<BulkResponse> releasingListener) {
-        threadPool.executor(Names.WRITE).execute(new ActionRunnable<>(releasingListener) {
+        threadPool.executor(executorName).execute(new ActionRunnable<>(releasingListener) {
             @Override
             protected void doRun() {
                 doInternalExecute(task, bulkRequest, executorName, releasingListener);