xfrm6_tunnel: 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>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>