workqueue: Make flush_workqueue() available again to non GPL modules

Commit 37b1ef31 ("workqueue: move
flush_scheduled_work() to workqueue.h") moved the exported non GPL
flush_scheduled_work() from a function to an inline wrapper.
Unfortunately, it directly calls flush_workqueue() which is a GPL function.
This has the effect of changing the licensing requirement for this function
and makes it unavailable to non GPL modules.

See commit ad7b1f84 ("workqueue: Make
schedule_work() available again to non GPL modules") for precedent.
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -2614,7 +2614,7 @@ void flush_workqueue(struct workqueue_struct *wq)
 out_unlock:
 	mutex_unlock(&wq->mutex);
 }
-EXPORT_SYMBOL_GPL(flush_workqueue);
+EXPORT_SYMBOL(flush_workqueue);
 
 /**
  * drain_workqueue - drain a workqueue