selftests/bpf: Add perfbuf multi-producer benchmark

selftests/bpf: Add perfbuf multi-producer benchmark

Add a multi-producer benchmark for perfbuf to complement the existing
ringbuf multi-producer test. Unlike ringbuf which uses a shared buffer
and experiences contention, perfbuf uses per-CPU buffers so the test
measures scaling behavior rather than contention.

This allows developers to compare perfbuf vs ringbuf performance under
multi-producer workloads when choosing between the two for their systems.

Signed-off-by: Gyutae Bae <gyutae.bae@navercorp.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260120090716.82927-1-gyutae.opensource@navercorp.com