backend: move freeing of td->mutex to main thread

backend: move freeing of td->mutex to main thread

On some platforms, we can end up freeing td->mutex while the wakee
is still inside fio_mutex_up(). This introduces a use-after-free
condition, and we crash.

Free the main thread_data mutex from the main thread instead,
when we know the process/thread is long dead and gone.

Signed-off-by: Jens Axboe <axboe@fb.com>