This is William Irwin's algorithmically O(1) version of
count_active_tasks (which is currently O(n) for n total tasks on the
system).

I like it a lot: we become O(1) because now we count uninterruptible
tasks, so we can return (nr_uninterruptible + nr_running).  It does not
introduce any overhead or hurt the case for small n, so I have no
complaints.

This copy has a small optimization over the original posting, but is
otherwise the same thing wli posted earlier.  I have tested to make sure
this returns accurate results and that the kernel profile improves.