ANDROID: cgroup: Use backport reservation for freeze time

cgroup v2 freeze time accounting added several new fields to struct
cgroup_freezer_state, which is enclosed by struct cgroup. Use the
backport reservation in struct cgroup to avoid the ABI breakage.

This change creates the auxiliary struct cgroup_kmi_ext_info and hides
the definition from gendwarfksyms to allow future modifications.

type 'struct cgroup' changed
  member 'u64 android_backport_reserved1' was removed
  member 'union { struct cgroup_kmi_ext_info* kmi_ext_info; struct { u64 android_backport_reserved1; }; union { }; }' was added

Bug: 404952324
Cc: T.J. Mercier <tjmercier@google.com>
Change-Id: Ib99ba06729ee40d7f06a981e89be6cd02d4faeeb
Signed-off-by: Tiffany Yang <ynaffit@google.com>