ip4_route_output_gtp
rt = ip4_route_output_gtp(&fl4, pctx->sk, pctx->peer.addr.s_addr,
rt = ip4_route_output_gtp(&fl4, sk, dst_ip, src_ip);
rt = ip4_route_output_gtp(&fl4, gtp->sk0, iph->saddr, iph->daddr);
rt = ip4_route_output_gtp(&fl4, gtp->sk1u, iph->saddr, iph->daddr);