selftests/bpf: Add BPF batch-timing library

Add a reusable timing library for BPF benchmarks that need to measure
BPF program execution time.

The BPF side (progs/bench_bpf_timing.bpf.h) provides pe

selftests/bpf: Add BPF batch-timing library

Add a reusable timing library for BPF benchmarks that need to measure
BPF program execution time.

The BPF side (progs/bench_bpf_timing.bpf.h) provides per-CPU sample
arrays and BENCH_BPF_LOOP(), a macro that brackets batch_iters
iterations with bpf_ktime_get_ns() reads and records the elapsed time.
One extra untimed iteration runs afterward for output validation.

The userspace side (benchs/bench_bpf_timing.c) collects samples from
the skeleton BSS, computes percentile statistics, and auto-calibrates
batch_iters to target ~10 ms per batch.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260427232313.1582588-3-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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