workqueue: make it clear that WQ_DRAINING is an internal flag

We're gonna add another internal WQ flag.  Let's make the distinction
clear.  Prefix WQ_DRAINING with __ and move it to bit 16.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index e152394..1751ec4 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -294,7 +294,7 @@
 	WQ_HIGHPRI		= 1 << 4, /* high priority */
 	WQ_CPU_INTENSIVE	= 1 << 5, /* cpu instensive workqueue */
 
-	WQ_DRAINING		= 1 << 6, /* internal: workqueue is draining */
+	__WQ_DRAINING		= 1 << 16, /* internal: workqueue is draining */
 
 	WQ_MAX_ACTIVE		= 512,	  /* I like 512, better ideas? */
 	WQ_MAX_UNBOUND_PER_CPU	= 4,	  /* 4 * #cpus for unbound wq */