cgroup: rstat: punt root-level optimization to individual controllers

Current users of the rstat code can source root-level statistics from
the native counters of their respective subsystem, allowing them to
forego aggregation at the root level.  This optimization is currently
implemented inside the generic rstat code, which doesn't track the root
cgroup and doesn't invoke the subsystem flush callbacks on it.

However, the memory controller cannot do this optimization, because
cgroup1 breaks out memory specifically for the local level, including at
the root level.  In preparation for the memory controller switching to
rstat, move the optimization from rstat core to the controllers.

Afterwards, rstat will always track the root cgroup for changes and
invoke the subsystem callbacks on it; and it's up to the subsystem to
special-case and skip aggregation of the root cgroup if it can source
this information through other, cheaper means.

This is the case for the io controller and the cgroup base stats.  In
th...