scheduler: Re-compute time-average nr_running on read

Re-compute time-average nr_running when it is read. This would
prevent reading stalled average value if there were no run-queue
changes for a long time. New average value is returned to the reader,
but not stored to avoid concurrent writes. Light-weight sequential
counter synchronization is used to assure data consistency for
re-computing average.

Change-Id: I8e4ea1b28ea00b3ddaf6ef7cdcd27866f87d360b