Use ifp->mtu6 in ospf6d and ripngd.
diff --git a/ripngd/ripng_nexthop.c b/ripngd/ripng_nexthop.c
index 2c5d45c..04b13c5 100644
--- a/ripngd/ripng_nexthop.c
+++ b/ripngd/ripng_nexthop.c
@@ -144,7 +144,7 @@
   stream_reset (s);
   num = 0;
 
-  mtu = ifp->mtu;
+  mtu = ifp->mtu6;
   if (mtu < 0)
     mtu = IFMINMTU;