selftests/bpf: Fix incorrect error checking for pthread_create

pthread_create returns 0 on success and a positive error code on failure;
it never returns a negative value. The current conditional branch can never be taken.
Failures during thread creation are silently ignored, which will lead to
invalid memory access when waiting on threads or dereferencing thread handles later.

Fixes: 91b2c0afd0 ("selftests/bpf: Add parallelism to test_progs")
Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Link: https://lore.kernel.org/bpf/20260723085100.482147-3-yangfeng59949@163.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

tools/testing/selftests/bpf/test_progs.c

@@ -1741,7 +1741,7 @@ static void server_main(void)
 		data[i].worker_id = i;
 		data[i].sock_fd = env.worker_socks[i];
 		rc = pthread_create(&dispatcher_threads[i], NULL, dispatch_thread, &data[i]);
-		if (rc < 0) {
+		if (rc) {
 			perror("Failed to launch dispatcher thread");
 			exit(EXIT_ERR_SETUP_INFRA);
 		}