hugetlb: add hugetlb.*.numa_stat file

For hugetlb backed jobs/VMs it's critical to understand the numa
information for the memory backing these jobs to deliver optimal
performance.

Currently this technically can be queried from /proc/self/numa_maps, but
there are significant issues with that.  Namely:

1. Memory can be mapped or unmapped.

2. numa_maps are per process and need to be aggregated across all
   processes in the cgroup.  For shared memory this is more involved as
   the userspace needs to make sure it doesn't double count shared
   mappings.

3. I believe querying numa_maps needs to hold the mmap_lock which adds
   to the contention on this lock.

For these reasons I propose simply adding hugetlb.*.numa_stat file,
   which shows the numa information of the cgroup similarly to
   memory.numa_stat.

On cgroup-v2:
   cat /sys/fs/cgroup/unified/test/hugetlb.2MB.numa_stat
   total=2097152 N0=2097152 N1=0

On cgroup-v1:
   cat /sys/fs/cgroup/hugetlb/test/hugetlb.2MB.numa_stat
   total=2097152 N0=2097152 N1=0
   hierarichal_total=2097152 N0=2097152 N1=0

This patch was tested manually by allocating hugetlb memory and querying
the hugetlb.*.numa_stat file of the cgroup and its parents.

[[email protected]: fix spelling mistake "hierarichal" -> "hierarchical"]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: fix copy/paste array assignment]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mina Almasry <[email protected]>
Signed-off-by: Colin Ian King <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reviewed-by: Muchun Song <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Jue Wang <[email protected]>
Cc: Yang Yao <[email protected]>
Cc: Joanna Li <[email protected]>
Cc: Cannon Matthews <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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