selftests/bpf: Add a test to verify previous stacksafe() fix

selftests/bpf: Add a test to verify previous stacksafe() fix

A selftest is added such that without the previous patch,
a crash can happen. With the previous patch, the test can
run successfully. The new test is written in a way which
mimics original crash case:
  main_prog
    static_prog_1
      static_prog_2
where static_prog_1 has different paths to static_prog_2
and some path has stack allocated and some other path
does not. A stacksafe() checking in static_prog_2()
triggered the crash.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240812214852.214037-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>