selftests/bpf: bpf_cookie: Make perf_event subtest trigger reliably

The perf_event subtest relies on SW_CPU_CLOCK sampling to trigger the BPF
program, but the current CPU burn loop can be too short on slower systems
and may fail to generate any overflow sample. This leaves pe_res unchanged
and makes the test flaky.

Make burn_cpu() take a loop count and use a longer burn only for the
perf_event subtest. Also scope perf_event_open() to the current task to
avoid wasting samples on unrelated activity.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20260228074555.122950-3-sun.jian.kdev@gmail.com

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