registering a notifier callback that listens to CPU up/down events to
modify the counters appropriately.

SGI-PV: 949726
SGI-Modid: xfs-linux-melb:xfs-kern:25214a
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NNathan Scott <nathans@sgi.com>