sched/fair: Don't move tasks to lower capacity cpus unless necessary

When lower capacity CPUs are load balancing and considering to pull
something from a higher capacity group, we should not pull tasks from a
cpu with only one task running as this is guaranteed to impede progress
for that task. If there is more than one task running, load balance in
the higher capacity group would have already made any possible moves to
resolve imbalance and we should make better use of system compute
capacity by moving a task if we still have more than one running.

Cc: valentin.schneider@arm.com
Cc: mingo@redhat.com
Cc: vincent.guittot@linaro.org
Cc: dietmar.eggemann@arm.com
Cc: gaku.inami.xh@renesas.com
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1530699470-29808-11-git-send-email-morten.rasmussen@arm.com