sched: Wrap rq->rd->overload accesses with READ/WRITE_ONCE

This variable can be read and set locklessly within update_sd_lb_stats().
As such, READ/WRITE_ONCE are added to make sure nothing terribly wrong
can happen because of the compiler.

Cc: vincent.guittot@linaro.org
Cc: mingo@redhat.com
Cc: dietmar.eggemann@arm.com
Cc: gaku.inami.xh@renesas.com
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1530699470-29808-9-git-send-email-morten.rasmussen@arm.com