From: David Ahern <redacted>
Date: Mon, 26 Feb 2018 13:20:27 -0700

+static void ip6_rt_init_dst(struct rt6_info *rt, struct rt6_info *ort)
+{
+	if (ort->rt6i_flags & RTF_REJECT) {
+		ip6_rt_init_dst_reject(rt, ort);
+		return;
+	}
+
+	rt->dst.error = 0;
+	rt->dst.output = ip6_output;
+
+...

So for reject routes we have the above helper which is basically a code
move from ip6_route_info_create.

For non-reject routes dst.error is 0 which is the rest of ip6_rt_init_dst.

My bad, thanks for explaining things to me.  The flag bit test above
completely escaped my eyes for some reason. :)