The unified hierarchy provides the cpu.stat file for every cgroup,
regardless if the CPU controller is enabled (in fact, setting the
systemd property CPUAccounting=True does not enable this controller
because of this fact). It provides the usage_usec, user_usec,
and system_usec by default. Instead of reading the stat for each
enabled controller (CPU and memory), just attempt to read them
each time the Stat() function is called.

Attempting to read the memory.stat file even if memory accounting
is not enabled seems insignificant (some other files always have
a read attempt, such as memory.current), and eliminates finding and
looping over enabled controllers.

Resolves: containerd#347

Signed-off-by: Jackson McKay  <[email protected]>