Revert "kernel-netlink: Don't fallback to peer address as gateway"

This reverts commit f717bb5249.

Causes issues in our testing environment. Default route via host is
preferred if no gateway is set in the installed routes.  Needs some
investigation.

References strongswan/strongswan#2548

src/libcharon/plugins/kernel_netlink/kernel_netlink_net.c

@@ -2075,6 +2075,10 @@ static host_t *get_route(private_kernel_netlink_net_t *this, host_t *dest,
 				*iface = get_interface_name_by_index(this, best->oif);
 			}
 		}
+		if (!addr && !match_net)
+		{	/* fallback to destination address */
+			addr = dest->clone(dest);
+		}
 	}
 	else
 	{