md/raid1/10: add missed blk plug

flush_pending_writes isn't always called with block plug, so add it, and plug
works in nested way.
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid1.c

@@ -809,11 +809,15 @@ static void flush_pending_writes(struct r1conf *conf)
 	spin_lock_irq(&conf->device_lock);
 
 	if (conf->pending_bio_list.head) {
+		struct blk_plug plug;
 		struct bio *bio;
+
 		bio = bio_list_get(&conf->pending_bio_list);
 		conf->pending_count = 0;
 		spin_unlock_irq(&conf->device_lock);
+		blk_start_plug(&plug);
 		flush_bio_list(conf, bio);
+		blk_finish_plug(&plug);
 	} else
 		spin_unlock_irq(&conf->device_lock);
 }

drivers/md/raid10.c

@@ -894,10 +894,13 @@ static void flush_pending_writes(struct r10conf *conf)
 	spin_lock_irq(&conf->device_lock);
 
 	if (conf->pending_bio_list.head) {
+		struct blk_plug plug;
 		struct bio *bio;
+
 		bio = bio_list_get(&conf->pending_bio_list);
 		conf->pending_count = 0;
 		spin_unlock_irq(&conf->device_lock);
+		blk_start_plug(&plug);
 		/* flush any pending bitmap writes to disk
 		 * before proceeding w/ I/O */
 		bitmap_unplug(conf->mddev->bitmap);
@@ -918,6 +921,7 @@ static void flush_pending_writes(struct r10conf *conf)
 				generic_make_request(bio);
 			bio = next;
 		}
+		blk_finish_plug(&plug);
 	} else
 		spin_unlock_irq(&conf->device_lock);
 }