sched: fix fair preempt check

Impact: fix cross-class preemption

Inter-class wakeup preemptions should go on class order.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 6167336..ebd6de8 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1329,6 +1329,9 @@
 		return;
 	}
 
+	if (unlikely(p->sched_class != &fair_sched_class))
+		return;
+
 	if (unlikely(se == pse))
 		return;