sched: initialize frequency domain cpumask

It's possible select_best_cpu() gets called before the first cpufreq
notifier call.  In such scenario select_best_cpu() can hang forever by
not clearing search_cpus.

Initialize frequency domain cpumask with the CPU of rq to avoid such
scenario.

CRs-fixed: 931349
Change-Id: If8d31c5477efe61ad7c6b336ba9e27ca6f556b63
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>