block: stack unplug

MD raid1 prepares to dispatch request in unplug callback. If make_request in
low level queue also uses unplug callback to dispatch request, the low level
queue's unplug callback will not be called. Recheck the callback list helps
this case.
Signed-off-by: NShaohua Li <shli@fusionio.com>
Signed-off-by: NNeilBrown <neilb@suse.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -2913,9 +2913,7 @@ static void flush_plug_callbacks(struct blk_plug *plug)
 {
 	LIST_HEAD(callbacks);
 
-	if (list_empty(&plug->cb_list))
-		return;
-
+	while (!list_empty(&plug->cb_list)) {
 		list_splice_init(&plug->cb_list, &callbacks);
 
 		while (!list_empty(&callbacks)) {
@@ -2925,6 +2923,7 @@ static void flush_plug_callbacks(struct blk_plug *plug)
 			list_del(&cb->list);
 			cb->callback(cb);
 		}
+	}
 }
 
 struct blk_plug_cb *blk_check_plugged(blk_plug_cb_fn unplug, void *data,