[IA64] correct file descriptor reference counting in perfmon (CVE-2006-3741)

Fix a bug in sys_perfmonctl() whereby it was not correctly
decrementing the file descriptor reference count.

Signed-off-by: Stephane Eranian <eranian@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>