sched/fair: prevent meaningless active migration

At present need_active_balance() determines whether active upmigration
is needed by using capacity_of().  CPU's capacity can be reduced by RT
tasks load therefore distinguishing cluster difference with
capacity_of() is wrong and leads suboptimal active migration within
same cluster.  Use capacity_orig_of() to distinguish different
clusters.

Change-Id: I3e1435246a8edc3ad618ef98a34866cfbd8c16a5
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>