selftests/bpf: Make bpf_util work on uniprocessor systems

The current implementation fails to work on uniprocessor systems.
Fix the parser to also handle the uniprocessor case.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>