QFreeList: fix memory order on block deletion

Blocks are likely to have been created in a differnt thread from the one
performing their deletion, so we need an acquire fence.

The rest of the atomics use in the class looks ok, but nevertheless warrants
a deeper analysis.

Change-Id: I1571ded3a06695b0d58b5bf1d80d6283ac21f959
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 6fa34930c23c7494a3f2703777f46794ff091e2b)
(cherry picked from commit 51bcc7e07e2bb5b42bb200dcd5269e9e9e2fe240)
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/tools/qfreelist_p.h

@@ -218,7 +218,7 @@ template <typename T, typename ConstantsType>
 inline QFreeList<T, ConstantsType>::~QFreeList()
 {
     for (int i = 0; i < ConstantsType::BlockCount; ++i)
-        delete [] _v[i].load();
+        delete [] _v[i].loadAcquire();
 }
 
 template <typename T, typename ConstantsType>