From: Martin Schwidefsky <schwidefsky@de.ibm.com>

Fix a race on sleep_avg in sched_exit().

The symptom I saw on 64-bit s390 has been a fixpoint divide exception
because sleep_avg had a value > NS_MAX_SLEEP_AVG.  I tracked it down and
the problem is sched_exit which recalculates the parents sleep average
without taking the runqueue lock.  schedule() subtracts run_time from
sleep_avg of the previous process.  This can turn out negative and is
corrected shortly after the subtraction but that is already too late.
sched_exit() already read the negative value an miscalculated the parents
sleep_avg -> bang.

I fixed this by adding task_rq_lock/task_rq_unlock to sched_exit().