sched/fair: Allow load bigger task load balance when nr_running is 2

When there is only 2 tasks in 1 cpu and the other
task is currently running, allow load bigger task
to be balanced if the other task is currently
running.

Change-Id: I489e9624ba010f9293272a67585e8209a786b787
Signed-off-by: Maria Yu <aiquny@codeaurora.org>