MEDIUM: tasks: No longer use rq.node.leaf_p as a lock.

MEDIUM: tasks: No longer use rq.node.leaf_p as a lock.

Now that we have the warranty that a task won't be added in the runqueue
while the TASK_QUEUED or the TASK_RUNNING flag is set, don't bother trying
to lock the task by setting leaf_p to 0x1 while inserting it in the runqueue
or having it in the tasklet_list, as nobody else will attempt to add it.