ANDROID: fix ABI-break in struct cgroup_root

Fix the ABI issues introduced by commit dd9542ae ("cgroup: Make
operations on the cgroup root_list RCU safe"). Use _some_ of the bits
reserved in cgroup_root for the new root->rcu member. This new offset
restricts the usage of kfree_rcu() though, so this call has replaced
with call_rcu() instead. Update the stg file with this update:

type 'struct cgroup_root' changed
  member 'u8 android_backport_reserved1[28]' was removed
  member 'union { struct callback_head rcu; struct { u8 android_backport_reserved1[28]; }; union { }; }' was added

Bug: 379227997
Suggested-by: T.J. Mercier <tjmercier@google.com>
Change-Id: I3e76ca05ee6d68f7167bf487348f438cd2a00d5f
Signed-off-by: Carlos Llamas <cmllamas@google.com>