Fix issue with td->mutex being used-after-free

Depending on how the OS schedules the threads, it's not necessarily
safe to immediately free a mutex after we have been woken up.
Remove the td->mutex freeing to normal cleanup after the job is done.

Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Jens Axboe <axboe@fb.com>