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.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>