hugetlb_cgroup: pass around @hugetlb_cgroup instead of @cgroup

cgroup controller API will be converted to primarily use struct
cgroup_subsys_state instead of struct cgroup.  In preparation, make
hugetlb_cgroup functions pass around struct hugetlb_cgroup instead of
struct cgroup.

This patch shouldn't cause any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>