selftests/bpf: Add bpf_for() benchmark

Add a bpf_for() benchmark modelled on bench_bpf_loop so the per-iteration
iterator cost can be measured and compared against bpf_loop. It runs an
empty bpf_for(i, 0, nr_loops) loop 1000 times per trigger and accounts
nr_loops hits per outer iteration:

  $ ./bench -p 1 --nr_loops 1000 bpf-for

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260804134601.2305303-7-puranjay@kernel.org