IPv4: RTM_GETROUTE: Add RTA_ENCAP to result

This patch adds an IPv4 routes encapsulation attribute
to the result of netlink RTM_GETROUTE requests
(e.g. ip route get 192.0.2.1).
Signed-off-by: Oliver Herms <oliver.peter.herms@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20201113085517.GA1307262@twsSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/route.c

@@ -2872,6 +2872,9 @@ static int rt_fill_info(struct net *net, __be32 dst, __be32 src,
 	if (rt->dst.dev &&
 	    nla_put_u32(skb, RTA_OIF, rt->dst.dev->ifindex))
 		goto nla_put_failure;
+	if (rt->dst.lwtstate &&
+	    lwtunnel_fill_encap(skb, rt->dst.lwtstate, RTA_ENCAP, RTA_ENCAP_TYPE) < 0)
+		goto nla_put_failure;
 #ifdef CONFIG_IP_ROUTE_CLASSID
 	if (rt->dst.tclassid &&
 	    nla_put_u32(skb, RTA_FLOW, rt->dst.tclassid))