From: Manfred Spraul <manfred@colorfullife.com>

de_thread calls list_del(&current->tasks), but current->tasks was never
added to the task list. The structure contains stale values from the parent.

switch_exec_pid() transforms a normal thread to a thread group leader.
Thread group leaders are included in the init_task.tasks linked list,
non-leaders are not in that list.  The patch adds the new thread group
leader to the linked list, otherwise de_thread corrupts the task list.