NFS: Micro-optimisation for nfs_wb_page()

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 966b8db..fab214a 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1516,6 +1516,8 @@
 		if (ret < 0)
 			goto out;
 	}
+	if (!PagePrivate(page))
+		return 0;
 	ret = nfs_sync_mapping_wait(page->mapping, &wbc, how);
 	if (ret >= 0)
 		return 0;