[IA64] sn2: add missing put_cpu()

sn_hwperf_op_cpu() uses get_cpu() but is missing a corresponding put_cpu().

Reported-by: Thomas Meyer <thomas@m3y3r.de>
Acked-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>