Merge branch 'selftests/bpf: Fixes for perf_buffer test'

Jiri Olsa says:

====================

hi,
sending fixes for perf_buffer test on systems
with offline cpus.

v2:
  - resend due to delivery issues, no changes

thanks,
jirka
Acked-by: John Fastabend <john.fastabend@gmail.com>
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>