FROMLIST: 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: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[from https://lore.kernel.org/lkml/1530699470-29808-9-git-send-email-morten.rasmussen@arm.com/

]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Change-Id: I14ce007f916bffd6a7938b7d56faf2b384d27887