sched: Prevent race conditions where upmigrate_min_nice changes

When upmigrate_min_nice is changed dec_nr_big_small_task() can trigger
BUG_ON(rq->nr_big_tasks < 0).  This happens when there is a task which was
considered as non-big task due to its nice > upmigrate_min_nice and later
upmigrate_min_nice is changed to higher value so the task becomes big task.
In this case runqueue still has nr_big_tasks = 0 incorrectly with current
implementation.  Consequently next scheduler tick sees a big task to
schedule and try to decrease nr_big_tasks which is already 0.

Introduce sched_upmigrate_min_nice which is updated atomically and re-count
the number of big and small tasks to fix BUG_ON() triggering.

Change-Id: I6f5fc62ed22bbe5c52ec71613082a6e64f406e58
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>