BUG/MEDIUM: tasks: make sure we pick all tasks in the run queue

BUG/MEDIUM: tasks: make sure we pick all tasks in the run queue

Commit 09eeb76 ("BUG/MEDIUM: tasks: Don't forget to increase/decrease
tasks_run_queue.") addressed a count issue in the run queue and uncovered
another issue with the way the tasks are dequeued from the global run
queue. The number of tasks to pick is computed using an integral divide,
which results in up to nbthread-1 tasks never being run. The fix simply
consists in getting rid of the divide and checking the task count in the
loop.

No backport is needed, this is 1.9-specific.