QSemaphore::release: Revert "Optimize cond var notification"

This reverts commit 60113056bc. Calling
    d->cond.notify_all();
without the mutex means that another thread could acquire the semaphore
(acquire the mutex, subtract from d->avail, return to caller) and
destroy it. That would mean this thread is now effectively dereferencing
a dangling d pointer.

Fixes: QTBUG-120762
Pick-to: 6.5 6.6 6.7
Change-Id: I196523f9addf41c2bf1ffffd17a96317f88b43dd
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>

src/corelib/thread/qsemaphore.cpp

@@ -401,10 +401,10 @@ void QSemaphore::release(int n)
         return;
     }
 
-    {
-        const auto locker = qt_scoped_lock(d->mutex);
-        d->avail += n;
-    }
+    // Keep mutex locked until after notify_all() lest another thread acquire()s
+    // the semaphore once d->avail == 0 and then destroys it, leaving `d` dangling.
+    const auto locker = qt_scoped_lock(d->mutex);
+    d->avail += n;
     d->cond.notify_all();
 }