[IA64] prevent ia64 from invoking irq handlers on offline CPUs

Make ia64 refrain from clearing a given to-be-offlined CPU's bit in the
cpu_online_mask until it has processed pending irqs.  This change
prevents other CPUs from being blindsided by an apparently offline CPU
nevertheless changing globally visible state.  Also remove the existing
redundant cpu_clear(cpu, cpu_online_map).

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>