sched/fair: Return -1 from find_energy_efficient_cpu() if no CPU fits

The root_domain overutilized flag is meant to be used as a disabling
condition for find_energy_efficient_cpu() (feec()). Since the
utilization of any running (or recently running) task is accounted in
a CPU's util_avg, not having the overutilized flag raised should ensure
there is at least one candidate (i.e. onto which the task fits) CPU in
feec() for any waking task.

There can however be cases where the overutilized flag hasn't been
raised yet, but feec() would not find a fitting CPU (fit_capacity())
and consequently return the previous CPU:

 * If the util estimation breaks the fit_capacity() condition because of
   a sudden higher task util_avg, the result would be a single bad
   placement.
   Only the next tick would raise the root_domain overutilized flag.

 * If that same condition is broken because of a uclamp.min value too
   high to fit any CPU, feec() would indefinitely wakeup on prev_cpu, as
   such a uclamp.min value wouldn't raise overutilized.

Detect such cases and fall-through to select_idle_capacity() by
returning -1 from feec().

Patch-header-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>