perf: Fix mmap_page::offset computation

There's multiple reason the counter might be unavailable, change the
condition to !->index since perf_event_index() should return 0 for all
those cases.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-1ixr3olci40w8rgv2evv2ldh@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -3268,7 +3268,7 @@ void perf_event_update_userpage(struct perf_event *event)
 	barrier();
 	userpg->index = perf_event_index(event);
 	userpg->offset = perf_event_count(event);
-	if (event->state == PERF_EVENT_STATE_ACTIVE)
+	if (userpg->index)
 		userpg->offset -= local64_read(&event->hw.prev_count);
 
 	userpg->time_enabled = enabled +