sched: do not balance on exec if SCHED_HMP

Rebalancing at exec time will currently undo any beneficial placement
that has been done during fork time, since select_best_cpu() will not
discount the currently running task.

For now just skip re-evaluating task placement at exec.

Change-Id: I1e5e0fcc329b7b53c338c8c73795ebd5e85a118b
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>