selftests/bpf: Check bpf_cubic_acked() is called via struct_ops

Test bpf_tcp_ca (in test_progs) checks multiple tcp_congestion_ops.
However, there isn't a test that verifies functions in the
tcp_congestion_ops is actually called. Add a check to verify that
bpf_cubic_acked is actually called during the test.

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230919060258.3237176-3-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c

@@ -185,6 +185,8 @@ static void test_cubic(void)
 
 	do_test("bpf_cubic", NULL);
 
+	ASSERT_EQ(cubic_skel->bss->bpf_cubic_acked_called, 1, "pkts_acked called");
+
 	bpf_link__destroy(link);
 	bpf_cubic__destroy(cubic_skel);
 }

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

@@ -490,6 +490,8 @@ static __always_inline void hystart_update(struct sock *sk, __u32 delay)
 	}
 }
 
+int bpf_cubic_acked_called = 0;
+
 void BPF_STRUCT_OPS(bpf_cubic_acked, struct sock *sk,
 		    const struct ack_sample *sample)
 {
@@ -497,6 +499,7 @@ void BPF_STRUCT_OPS(bpf_cubic_acked, struct sock *sk,
 	struct bictcp *ca = inet_csk_ca(sk);
 	__u32 delay;
 
+	bpf_cubic_acked_called = 1;
 	/* Some calls are for duplicates without timetamps */
 	if (sample->rtt_us < 0)
 		return;