sched: Fix bug in average nr_running and nr_iowait calculation

sched_get_nr_running_avg() returns average nr_running and nr_iowait
task count since it was last invoked. Fix several bugs in their
calculation.

* sched_update_nr_prod() needs to consider that nr_running count can
  change by more than 1 when CFS_BANDWIDTH feature is used

* sched_get_nr_running_avg() needs to sum up nr_iowait count across
  all cpus, rather than just one

* sched_get_nr_running_avg() could race with sched_update_nr_prod(),
  as a result of which it could use curr_time which is behind a cpu's
  'last_time' value. That would lead to erroneous calculation of
  average nr_running or nr_iowait.

While at it, fix also a bug in BUG_ON() check in
sched_update_nr_prod() function and remove unnecessary nr_running
argument to sched_update_nr_prod() function.

Change-Id: I46737614737292fae0d7204c4648fb9b862f65b2
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>