Oprofile: call model->shutdown() only once to avoid calling release_ev*()
multiple times

[ tglx: arch/x86 adaptation ]
Signed-off-by: NStephane Eranian <eranian@hpl.hp.com>
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>