[IA64] save and restore cpus_allowed in cpu_idle_wait

Save and Restore the task's cpus_allowed mask, across the set_cpus_allowed()
in cpu_idle_wait(). Without this, we will endup corrupting task's cpu affinity.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>