selftests/bpf: Add attach bench test

Adding test that reads all functions from ftrace available_filter_functions
file and attach them all through kprobe_multi API.

It also prints stats info with -v option, like on my setup:

  test_bench_attach: found 48712 functions
  test_bench_attach: attached in   1.069s
  test_bench_attach: detached in   0.373s

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220510122616.2652285-6-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>