perf_counter: fix hrtimer sampling

Impact: fix deadlock with perfstat

Fix for the perfstat fubar..

We cannot unconditionally call hrtimer_cancel() without ever having done
hrtimer_init() on the thing.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Orig-LKML-Reference: <1236959027.22447.149.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>