ia64: Use preempt_schedule_irq

Use the proper core function instead of fiddling with PREEMPT_ACTIVE
and enable/disable interrupts in the low level code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: linux-ia64@vger.kernel.org
Link: http://lkml.kernel.org/r/20130917183628.857145384@linutronix.de