perf: Fix SIGIO handling

Vince noticed that unless we mmap() a buffer, SIGIO gets lost. So
explicitly push the wakeup (including signals) when requested.

Reported-by: Vince Weaver <vweaver1@eecs.utk.edu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org>
Link: http://lkml.kernel.org/n/tip-2euus3f3x3dyvdk52cjxw8zu@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -5028,6 +5028,14 @@ static int __perf_event_overflow(struct perf_event *event, int nmi,
 	else
 		perf_event_output(event, nmi, data, regs);
 
+	if (event->fasync && event->pending_kill) {
+		if (nmi) {
+			event->pending_wakeup = 1;
+			irq_work_queue(&event->pending);
+		} else
+			perf_event_wakeup(event);
+	}
+
 	return ret;
 }