cred: Use KMEM_CACHE() instead of kmem_cache_create()

Commit 0a31bd5f ("KMEM_CACHE(): simplify slab cache creation")
introduces a new macro. Use the new KMEM_CACHE() macro instead of
direct kmem_cache_create() to simplify the creation of SLAB caches.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
[PM: alignment fixes in both code and description]
Signed-off-by: Paul Moore <paul@paul-moore.com>