selinux: replace kmem_cache_create() with KMEM_CACHE()

selinux: replace kmem_cache_create() with KMEM_CACHE()

Based on guidance in include/linux/slab.h, replace kmem_cache_create()
with KMEM_CACHE() for sources under security/selinux to simplify creation
of SLAB caches.

Signed-off-by: Eric Suen <ericsu@linux.microsoft.com>
[PM: minor grammar nits in the description]
Signed-off-by: Paul Moore <paul@paul-moore.com>