libbpf: fix max() type mismatch for 32bit

It fixes build error for 32bit caused by type mismatch
size_t/unsigned long.

Fixes: bf829271

 ("libbpf: refactor map initialization")
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>