bpf: Add open coded version of kmem_cache iterator

bpf: Add open coded version of kmem_cache iterator

Add a new open coded iterator for kmem_cache which can be called from a
BPF program like below.  It doesn't take any argument and traverses all
kmem_cache entries.

  struct kmem_cache *pos;

  bpf_for_each(kmem_cache, pos) {
      ...
  }

As it needs to grab slab_mutex, it should be called from sleepable BPF
programs only.

Also update the existing iterator code to use the open coded version
internally as suggested by Andrii.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20241030222819.1800667-1-namhyung@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>