i386: make Oprofile call shutdown() only once per session

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

[ tglx: arch/x86 adaptation ]

Signed-off-by: Stephane Eranian <eranian@hpl.hp.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>