sched: iterate search CPUs starting from prev_cpu for optimization

Function best_small_task_cpu() looks for a mostly idle CPU and returns it
as the best CPU for a given small task.  At present, however, it cannot
break the CPU search loop when the function found a mostly idle CPU but
continues to iterate CPU search loop because the function needs to find
and return the given task's previous CPU as the best CPU to avoid
unnecessary task migration when the previous CPU is mostly idle.

Optimize the function best_small_task_cpu() to iterate search CPUs
starting from the given task's CPU so it can break the loop as soon as
mostly idle CPU found.  This optimization saves few hundreds ns spent by
the function and doesn't make any functional change.

CRs-fixed: 849655
Change-Id: I8c540963487f4102dac4d54e9f98e24a4a92a7b3
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>