selftests/bpf: Fix perf_buffer test on systems w/ offline CPUs

Fix up perf_buffer.c selftest to take into account offline/missing CPUs.

Fixes: ee5cf82c

 ("selftests/bpf: test perf buffer API")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191212013621.1691858-1-andriin@fb.com