selftests/bpf: Add selftest for check_stack_max_depth bug

Use the bpf_timer_set_callback helper to mark timer_cb as an async
callback, and put a direct call to timer_cb in the main subprog.

As the check_stack_max_depth happens after the do_check pass, the order
does not matter. Without the previous fix, the test passes successfully.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230705144730.235802-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+
+#include "async_stack_depth.skel.h"
+
+void test_async_stack_depth(void)
+{
+	RUN_TESTS(async_stack_depth);
+}