selftests/bpf: Fix xdp_synproxy build failure if CONFIG_NF_CONNTRACK=m/n

When CONFIG_NF_CONNTRACK=m, struct bpf_ct_opts and enum member
BPF_F_CURRENT_NETNS are not exposed. This commit allows building the
xdp_synproxy selftest in such cases. Note that nf_conntrack must be
loaded before running the test if it's compiled as a module.

This commit also allows this selftest to be successfully compiled when
CONFIG_NF_CONNTRACK is disabled.

One unused local variable of type struct bpf_ct_opts is also removed.

Fixes: fb5cd0ce70 ("selftests/bpf: Add selftests for raw syncookie helpers")
Reported-by: Yauheni Kaliuta <ykaliuta@redhat.com>
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220708130319.1016294-1-maximmi@nvidia.com

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

@@ -77,16 +77,30 @@ struct {
 	__uint(max_entries, MAX_ALLOWED_PORTS);
 } allowed_ports SEC(".maps");
 
+/* Some symbols defined in net/netfilter/nf_conntrack_bpf.c are unavailable in
+ * vmlinux.h if CONFIG_NF_CONNTRACK=m, so they are redefined locally.
+ */
+
+struct bpf_ct_opts___local {
+	s32 netns_id;
+	s32 error;
+	u8 l4proto;
+	u8 dir;
+	u8 reserved[2];
+} __attribute__((preserve_access_index));
+
+#define BPF_F_CURRENT_NETNS (-1)
+
 extern struct nf_conn *bpf_xdp_ct_lookup(struct xdp_md *xdp_ctx,
 					 struct bpf_sock_tuple *bpf_tuple,
 					 __u32 len_tuple,
-					 struct bpf_ct_opts *opts,
+					 struct bpf_ct_opts___local *opts,
 					 __u32 len_opts) __ksym;
 
 extern struct nf_conn *bpf_skb_ct_lookup(struct __sk_buff *skb_ctx,
 					 struct bpf_sock_tuple *bpf_tuple,
 					 u32 len_tuple,
-					 struct bpf_ct_opts *opts,
+					 struct bpf_ct_opts___local *opts,
 					 u32 len_opts) __ksym;
 
 extern void bpf_ct_release(struct nf_conn *ct) __ksym;
@@ -393,7 +407,7 @@ static __always_inline int tcp_dissect(void *data, void *data_end,
 
 static __always_inline int tcp_lookup(void *ctx, struct header_pointers *hdr, bool xdp)
 {
-	struct bpf_ct_opts ct_lookup_opts = {
+	struct bpf_ct_opts___local ct_lookup_opts = {
 		.netns_id = BPF_F_CURRENT_NETNS,
 		.l4proto = IPPROTO_TCP,
 	};
@@ -714,10 +728,6 @@ static __always_inline int syncookie_handle_ack(struct header_pointers *hdr)
 static __always_inline int syncookie_part1(void *ctx, void *data, void *data_end,
 					   struct header_pointers *hdr, bool xdp)
 {
-	struct bpf_ct_opts ct_lookup_opts = {
-		.netns_id = BPF_F_CURRENT_NETNS,
-		.l4proto = IPPROTO_TCP,
-	};
 	int ret;
 
 	ret = tcp_dissect(data, data_end, hdr);