selftests/bpf: fix compilation failure when CONFIG_NF_FLOW_TABLE=m

In many cases, kernel netfilter functionality is built as modules.
If CONFIG_NF_FLOW_TABLE=m in particular, progs/xdp_flowtable.c
(and hence selftests) will fail to compile, so add a ___local
version of "struct flow_ports".

Fixes: c77e572d3a8c ("selftests/bpf: Add selftest for bpf_xdp_flow_lookup kfunc")
Signed-off-by: Alan Maguire <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/progs/xdp_flowtable.c

···
 	return true;
 }
 
+struct flow_ports___local {
+	__be16 source, dest;
+} __attribute__((preserve_access_index));
+
 SEC("xdp.frags")
 int xdp_flowtable_do_lookup(struct xdp_md *ctx)
 {
···
 	};
 	void *data = (void *)(long)ctx->data;
 	struct ethhdr *eth = data;
-	struct flow_ports *ports;
+	struct flow_ports___local *ports;
 	__u32 *val, key = 0;
 
 	if (eth + 1 > data_end)
···
 	case bpf_htons(ETH_P_IP): {
 		struct iphdr *iph = data + sizeof(*eth);
 
-		ports = (struct flow_ports *)(iph + 1);
+		ports = (struct flow_ports___local *)(iph + 1);
 		if (ports + 1 > data_end)
 			return XDP_PASS;
 
···
 		struct in6_addr *dst = (struct in6_addr *)tuple.ipv6_dst;
 		struct ipv6hdr *ip6h = data + sizeof(*eth);
 
-		ports = (struct flow_ports *)(ip6h + 1);
+		ports = (struct flow_ports___local *)(ip6h + 1);
 		if (ports + 1 > data_end)
 			return XDP_PASS;