bpf/tests: Use struct_size()

Use struct_size() instead of hand writing it. This is less verbose and
more informative.

Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230531043251.989312-1-suhui@nfschina.com