xfrm: Simplify the allocation of slab caches in xfrm_policy_init

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>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>