netfilter: nf_conncount: 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: Florian Westphal <fw@strlen.de>