sched: Optimize the select_best_cpu() "for" loop

select_best_cpu() is agnostic of the hardware topology. This means that
certain functions such as task_will_fit() and skip_cpu() are run
unnecessarily for every CPU in a cluster whereas they need to run only
once per cluster. Reduce the execution time of select_best_cpu() by
ensuring these functions run only once per cluster. The frequency domain
mask is used to identify CPUs that fall in the same cluster.

CRs-fixed: 849655
Change-Id: Id24208710a0fc6321e24d9a773f00be9312b75de
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>
[joonwoop@codeaurora.org: added continue after clearing search_cpus.
 fixed indentations with space.  fixed skip_cpu() to return true when rq ==
 task_rq.]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>