hugetlbfs: add a list for tracking in-use HugeTLB pages
hugepage_activelist will be used to track currently used HugeTLB pages.
We need to find the in-use HugeTLB pages to support memcg removal. On
memcg removal we update the page's memory cgroup to point to parent
cgroup.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Hillf Danton <dhillf@gmail.com> Cc: Michal Hocko <mhocko@suse.cz> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>