selftests: bpf: don't depend on hardcoded perf sample_freq

When running stacktrace_build_id_nmi, try to query
kernel.perf_event_max_sample_rate sysctl and use it as a sample_freq.
If there was an error reading sysctl, fallback to 5000.

kernel.perf_event_max_sample_rate sysctl can drift and/or can be
adjusted by the perf tool, so assuming a fixed number might be
problematic on a long running machine.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>