Btrfs: make ordered extent be flushed by multi-task

Though the process of the ordered extents is a bit different with the delalloc inode
flush, but we can see it as a subset of the delalloc inode flush, so we also handle
them by flush workers.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h
index e8dcec6..efc7c29 100644
--- a/fs/btrfs/ordered-data.h
+++ b/fs/btrfs/ordered-data.h
@@ -128,8 +128,11 @@
 	struct list_head root_extent_list;
 
 	struct btrfs_work work;
-};
 
+	struct completion completion;
+	struct btrfs_work flush_work;
+	struct list_head work_list;
+};
 
 /*
  * calculates the total size you need to allocate for an ordered sum