selftests/bpf: add test for nullable PTR_TO_BUF access

selftests/bpf: add test for nullable PTR_TO_BUF access

Add iter_buf_null_fail with two tests and a test runner:
  - iter_buf_null_deref: verifier must reject direct dereference of
    ctx->key (PTR_TO_BUF | PTR_MAYBE_NULL) without a null check
  - iter_buf_null_check_ok: verifier must accept dereference after
    an explicit null check

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
Link: https://lore.kernel.org/r/20260407145421.4315-1-tpluszz77@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>