selftests/bpf: Add a test for open coded kmem_cache iter

selftests/bpf: Add a test for open coded kmem_cache iter

The new subtest runs with bpf_prog_test_run_opts() as a syscall prog.
It iterates the kmem_cache using bpf_for_each loop and count the number
of entries.  Finally it checks it with the number of entries from the
regular iterator.

  $ ./vmtest.sh -- ./test_progs -t kmem_cache_iter
  ...
  #130/1   kmem_cache_iter/check_task_struct:OK
  #130/2   kmem_cache_iter/check_slabinfo:OK
  #130/3   kmem_cache_iter/open_coded_iter:OK
  #130     kmem_cache_iter:OK
  Summary: 1/3 PASSED, 0 SKIPPED, 0 FAILED

Also simplify the code by using attach routine of the skeleton.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20241030222819.1800667-2-namhyung@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>