writeback: make bdi_start_background_writeback() take bdi_writeback instead of backing_dev_info

bdi_start_background_writeback() currently takes @bdi and kicks the
root wb (bdi_writeback).  In preparation for cgroup writeback support,
make it take wb instead.

This patch doesn't make any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 45baf6c..92aaf64 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -228,23 +228,23 @@
 }
 
 /**
- * bdi_start_background_writeback - start background writeback
- * @bdi: the backing device to write from
+ * wb_start_background_writeback - start background writeback
+ * @wb: bdi_writback to write from
  *
  * Description:
  *   This makes sure WB_SYNC_NONE background writeback happens. When
- *   this function returns, it is only guaranteed that for given BDI
+ *   this function returns, it is only guaranteed that for given wb
  *   some IO is happening if we are over background dirty threshold.
  *   Caller need not hold sb s_umount semaphore.
  */
-void bdi_start_background_writeback(struct backing_dev_info *bdi)
+void wb_start_background_writeback(struct bdi_writeback *wb)
 {
 	/*
 	 * We just wake up the flusher thread. It will perform background
 	 * writeback as soon as there is no other work to do.
 	 */
-	trace_writeback_wake_background(bdi);
-	wb_wakeup(&bdi->wb);
+	trace_writeback_wake_background(wb->bdi);
+	wb_wakeup(wb);
 }
 
 /*