mutex: add __fio_mutex_remove()

We have to remember to free the condvar, so add a __fio_mutex_remove()
for cases that init a mutex inside an smalloc'ed region with
__fio_mutex_init().

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/server.c b/server.c
index ede291f..3171979 100644
--- a/server.c
+++ b/server.c
@@ -1431,6 +1431,7 @@
 	*datap = data;
 
 	sfree(rep->data);
+	__fio_mutex_remove(&rep->lock);
 	sfree(rep);
 	return 0;
 }