selftests/bpf: Do not fail build if CONFIG_NF_CONNTRACK=m/n

Some users have complained that selftests fail to build when
CONFIG_NF_CONNTRACK=m. It would be useful to allow building as long as
it is set to module or built-in, even though in case of building as
module, user would need to load it before running the selftest. Note
that this also allows building selftest when CONFIG_NF_CONNTRACK is
disabled.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220120164932.2798544-1-memxor@gmail.com

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

@@ -17,18 +17,27 @@ int test_enonet_netns_id = 0;
 int test_enoent_lookup = 0;
 int test_eafnosupport = 0;
 
+struct nf_conn;
+
+struct bpf_ct_opts___local {
+	s32 netns_id;
+	s32 error;
+	u8 l4proto;
+	u8 reserved[3];
+} __attribute__((preserve_access_index));
+
 struct nf_conn *bpf_xdp_ct_lookup(struct xdp_md *, struct bpf_sock_tuple *, u32,
-				  struct bpf_ct_opts *, u32) __ksym;
+				  struct bpf_ct_opts___local *, u32) __ksym;
 struct nf_conn *bpf_skb_ct_lookup(struct __sk_buff *, struct bpf_sock_tuple *, u32,
-				  struct bpf_ct_opts *, u32) __ksym;
+				  struct bpf_ct_opts___local *, u32) __ksym;
 void bpf_ct_release(struct nf_conn *) __ksym;
 
 static __always_inline void
 nf_ct_test(struct nf_conn *(*func)(void *, struct bpf_sock_tuple *, u32,
-				   struct bpf_ct_opts *, u32),
+				   struct bpf_ct_opts___local *, u32),
 	   void *ctx)
 {
-	struct bpf_ct_opts opts_def = { .l4proto = IPPROTO_TCP, .netns_id = -1 };
+	struct bpf_ct_opts___local opts_def = { .l4proto = IPPROTO_TCP, .netns_id = -1 };
 	struct bpf_sock_tuple bpf_tuple;
 	struct nf_conn *ct;