selftests/bpf: Fix possible hang in sockopt_inherit

Andrii reported that sockopt_inherit occasionally hangs up on 5.5 kernel [0].
This can happen if server_thread runs faster than the main thread.
In that case, pthread_cond_wait will wait forever because
pthread_cond_signal was executed before the main thread was blocking.
Let's move pthread_mutex_lock up a bit to make sure server_thread
runs strictly after the main thread goes to sleep.

(Not sure why this is 5.5 specific, maybe scheduling is less
deterministic? But I was able to confirm that it does indeed
happen in a VM.)

[0] https://lore.kernel.org/bpf/CAEf4BzY0-bVNHmCkMFPgObs=isUAyg-dFzGDY7QWYkmm7rmTSg@mail.gmail.com/Reported-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200715224107.3591967-1-sdf@google.com

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

@@ -193,11 +193,10 @@ static void run_test(int cgroup_fd)
 	if (CHECK_FAIL(server_fd < 0))
 		goto close_bpf_object;
 
+	pthread_mutex_lock(&server_started_mtx);
 	if (CHECK_FAIL(pthread_create(&tid, NULL, server_thread,
 				      (void *)&server_fd)))
 		goto close_server_fd;
-
-	pthread_mutex_lock(&server_started_mtx);
 	pthread_cond_wait(&server_started, &server_started_mtx);
 	pthread_mutex_unlock(&server_started_mtx);