writeback: introduce wbc.for_background

It will lower the flush priority for NFS, and maybe more in future.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/nfs/write.c

@@ -178,7 +178,7 @@ static int wb_priority(struct writeback_control *wbc)
 {
 	if (wbc->for_reclaim)
 		return FLUSH_HIGHPRI | FLUSH_STABLE;
-	if (wbc->for_kupdate)
+	if (wbc->for_kupdate || wbc->for_background)
 		return FLUSH_LOWPRI;
 	return 0;
 }