[XFS] Add support for hotplug CPUs to the per-CPU superblock counters by

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: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>