selftests/bpf: avoid reporting +100% difference in veristat for actual 0%

In special case when both base and comparison values are 0, veristat
currently reports "+0 (+100%)" difference, which is quite confusing. Fix
it up to be "+0 (+0%)".

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221005161450.1064469-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>