BUG/MAJOR: sched: make sure task_kill() always queues the task

BUG/MAJOR: sched: make sure task_kill() always queues the task

task_kill() may fail to queue a task if this task has never ever run,
because its equivalent (tasklet->list) member has never been "emptied"
since it didn't pass through the LIST_DEL_INIT() that's performed by
run_tasks_from_lists(). This results in these tasks to never be freed.

It happens during the mux takeover since the target task usually is
the timeout task which, by definition, has never run yet.

This fixes commit eb8c2c69f ("MEDIUM: sched: implement task_kill() to
kill a task") which was introduced after 2.2-dev11 and doesn't need to
be backported.