BPF_PROG_TEST_RUN does not reset output buffer on each run

[lmb]

It's very easy to mess up benchmarks when using Program.Benchmark, since the kernel feeds the output from benchmark run N to run N+1. This means we don't measure the exact same action on each iteration.

Ideally the kernel would fix this, but I'm not sure that is going to happen. I propose that Benchmark() checks that input == output, and raises an error otherwise.

[lmb]

One thing I realised: programs can also modify maps, just checking the input vs output doesn't cover that. I don't think it's feasible to do so either.