[IA64] hotplug/ia64: SN Hotplug Driver: SN IRQ Fixes

This patch  fixes the SN IRQ code such that cpu affinity and
Hotplug can modify IRQ values.  The sn_irq_info structures are now locked
using a RCU lock mechanism to avoid lock contention in the lost interrupt
WAR code.

Signed-off-by: Prarit Bhargava <prarit@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>