block: optimise blk_flush_plug_list

Don't call flush_plug_callbacks if there are no plug callbacks.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
[hch: split from a larger patch]
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211020144119.142582-4-hch@lst.deSigned-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1639,8 +1639,8 @@ EXPORT_SYMBOL(blk_check_plugged);
 
 void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
-	flush_plug_callbacks(plug, from_schedule);
-
+	if (!list_empty(&plug->cb_list))
+		flush_plug_callbacks(plug, from_schedule);
 	if (!rq_list_empty(plug->mq_list))
 		blk_mq_flush_plug_list(plug, from_schedule);
 	if (unlikely(!from_schedule && plug->cached_rq))