smart: sched: find_idlest_cpu() shouldn't return -1

The find_idlest_cpu() function can return -1 if the sched_group
consists of only one CPU and it's skipped by cpu_allowed_for_cfs().
In this case, select_task_rq_fair() can return -1 that leads to
page handling request failure in sched_exec().

Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>