task_finder2: fix task worker race on module unload

task_finder2: fix task worker race on module unload

Unfortunately, __stp_tf_cancel_all_task_work() does not guarantee that
all of the task finder's task workers will be finished executing when it
returns. In this case, we rely on the stp_task_work API to prevent the
module from being unloaded while there are task workers in-flight, which
works, but the stp_task_work API is notified of a task worker finishing
before it actually finishes. Inside __stp_tf_task_worker_fn(), the
call to the task worker's function (tf_work->func) is where the final
refcount in the stp_task_work API could be put, but there will still be
instructions left in the task worker that will be executing for a short
time after that. In that short time, there can be a race where the
module is unloaded before the task worker finishes executing all of its
instructions, especially if the task worker gets preempted during this
time on a PREEMPT kernel.

To remedy this, we must ensure that the last instruction in
__stp_tf_task_worker_fn() is where the stp_task_work API is notified of
a task worker finishing.