mutex: ensure that fio_mutex_up() holds mutex lock during wakeup

mutex: ensure that fio_mutex_up() holds mutex lock during wakeup

Some implementations mandate that you hold the lock while
issuing the wakeup, as not to lose a wakeup. This seems to
be the case for Windows. See the lengthy thread on the topic
here:

https://www.spinics.net/lists/fio/msg06772.html

Thanks to Sitsofe Wheeler for heroic debugging.

Reported-by: Rebecca Cran <rebecca@bluestop.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>