writeback: mark background writeback as such

If we're doing background type writes, then use the appropriate
background write flags for that.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 50c96ee..c78f9f0 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -107,6 +107,8 @@
 {
 	if (wbc->sync_mode == WB_SYNC_ALL)
 		return REQ_SYNC;
+	else if (wbc->for_kupdate || wbc->for_background)
+		return REQ_BACKGROUND;
 
 	return 0;
 }