nullfs: dirty v_object must imply the need for inactivation

Otherwise pages are cleaned some time later when the lower fs decides
that it is time to do it.  This mostly manifests itself as delayed
mtime update, e.g. breaking make-like programs.

Reported by:	mav
Tested by:	mav, pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/fs/nullfs/null_vnops.c

@@ -818,7 +818,7 @@ static int
 null_need_inactive(struct vop_need_inactive_args *ap)
 {
 
-	return (null_want_recycle(ap->a_vp));
+	return (null_want_recycle(ap->a_vp) || vn_need_pageq_flush(ap->a_vp));
 }
 
 /*