audit: use KMEM_CACHE() instead of kmem_cache_create()

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: subject line tweaks]
Signed-off-by: Paul Moore <paul@paul-moore.com>