Optimize cond var notification

Notification of a conditional variable shouldn't be under the locked
mutex, as it may affect extra mutex contentions.

Pick-to: 6.5 6.6
Change-Id: Ie8429eca3f36e9a6e8e5ad2e0337bbf508f5b326
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qsemaphore.cpp

@@ -400,8 +400,10 @@ void QSemaphore::release(int n)
         return;
     }
 
+    {
         const auto locker = qt_scoped_lock(d->mutex);
         d->avail += n;
+    }
     d->cond.notify_all();
 }