sched: Reassign prev and switch_count when reacquire_kernel_lock() fail

Assume A->B schedule is processing, if B have acquired BKL before and it
need reschedule this time. Then on B's context, it will go to
need_resched_nonpreemptible for reschedule. But at this time, prev and
switch_count are related to A. It's wrong and will lead to incorrect
scheduler statistics.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <2674af741001102238w7b0ddcadref00d345e2181d11@mail.gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -5530,8 +5530,11 @@ need_resched_nonpreemptible:
 
 	post_schedule(rq);
 
-	if (unlikely(reacquire_kernel_lock(current) < 0))
+	if (unlikely(reacquire_kernel_lock(current) < 0)) {
+		prev = rq->curr;
+		switch_count = &prev->nivcsw;
 		goto need_resched_nonpreemptible;
+	}
 
 	preempt_enable_no_resched();
 	if (need_resched())