MINOR: tasks: Introduce __task_get_current_owner

MINOR: tasks: Introduce __task_get_current_owner

Introduce a new function, __task_get_current_owner, that returns the
owner of a task based on its current tid.
-1 means there is no current owner, otherwise either the tid is >= 0, in
which case it will just return it, or it's < -1, in which case it will
return -2 - tid, the tid of the thread with the current ownership.