fs: make fsync_buffers_list() plug

It used WRITE_SYNC_PLUG before and potentially submits a batch
of IO, so lets enable plugging for this case.

Signed-off-by: Jens Axboe <[e-mail]>

fs/buffer.c

···
 	struct list_head tmp;
 	struct address_space *mapping;
 	int err = 0, err2;
+	struct blk_plug plug;
 
 	INIT_LIST_HEAD(&tmp);
+	blk_start_plug(&plug);
 
 	spin_lock(lock);
 	while (!list_empty(list)) {
···
 			}
 		}
 	}
+
+	spin_unlock(lock);
+	blk_finish_plug(&plug);
+	spin_lock(lock);
 
 	while (!list_empty(&tmp)) {
 		bh = BH_ENTRY(tmp.prev);