ipv4: redirect dst output to lwtunnel output

For input routes with tunnel encap state this patch redirects
dst output functions to lwtunnel_output which later resolves to
the corresponding lwtunnel output function.

This has been tested to work with mpls ip tunnels.
Signed-off-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1633,6 +1633,8 @@ static int __mkroute_input(struct sk_buff *skb,
 	rth->dst.output = ip_output;
 
 	rt_set_nexthop(rth, daddr, res, fnhe, res->fi, res->type, itag);
+	if (lwtunnel_output_redirect(rth->rt_lwtstate))
+		rth->dst.output = lwtunnel_output;
 	skb_dst_set(skb, &rth->dst);
 out:
 	err = 0;