sched: Reset rq->next_interval before going idle

next_balance, the point in jiffy time scale when a cpu will next load
balance, could have been calculated when the cpu was busy. A busy cpu
will apply its sched domain's busy_factor (usually > 1) in computing
next_balance for that sched domain, which causes the (busy) cpu to load
balance less frequently in its sched domains. However when the same cpu
is going idle, its next_balance needs to be reset without consideration
of busy_factor. Failure to do so would not trigger nohz idle balancer on
that cpu for unnecessarily long time (introducing additional scheduling
latencies for tasks). Fix bug in scheduler which aims to reset
next_balance before a cpu goes idle (as per existing comment) but is
clearly not doing so.

Change-Id: I7e027a51686528c4092d770c7d33c874d38f5df4
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>