nexus: Fix might_sleep() assertion on CPU hotplug
nexus: Fix might_sleep() assertion on CPU hotplug

- The CPU hotplug notifier is now sometimes called from atomic context,
but only for notifications we don't actually care about.  (This occurs
on >= 2.6.23 for hot-remove and >= 2.6.28 for hot-add.)  Therefore, only
take the thread mutex when receiving a notification we intend to process.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr