selftests/bpf: Clean up sys_nanosleep uses

This patch cleans up a few things:

  * dynptr_fail.c:
    There is no sys_nanosleep tracepoint. dynptr_fail only tests
    that the prog load fails, so just SEC("?raw_tp") suffices here.

  * test_bpf_cookie:
    There is no sys_nanosleep kprobe. The prog is loaded in
    userspace through bpf_program__attach_kprobe_opts passing in
    SYS_NANOSLEEP_KPROBE_NAME, so just SEC("k{ret}probe") suffices here.

  * test_helper_restricted:
    There is no sys_nanosleep kprobe. test_helper_restricted only tests
    that the prog load fails, so just SEC("?kprobe")( suffices here.

There are no functional changes.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220805171405.2272103-1-joannelkoong@gmail.com