QFreeList: fix UB (data race) in release()

The load from _v[block] must acquire, to match the storeRelease()
implied in the testAndSetRelease() in next(), where _v[block] was
allocated. A loadReleaxed() will not establish the happens-before
relation required to avoid a data race.

So use loadAcquire().

Pick-to: 6.5
Change-Id: I34003ef7510a0d78f7851d2b4b4aab01a686babf
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit b6faf63995b63d5958cf4b95ad1e9bab17d1bc6d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 6b89694a016474c7877ae07f0d49efd42f8fd9ec)
(cherry picked from commit 51f2979e5f79e1d67425834167487d41bb59f9d7)

src/corelib/tools/qfreelist_p.h

@@ -232,7 +232,7 @@ inline void QFreeList<T, ConstantsType>::release(int id)
 {
     int at = id & ConstantsType::IndexMask;
     const int block = blockfor(at);
-    ElementType *v = _v[block].loadRelaxed();
+    ElementType *v = _v[block].loadAcquire();
 
     int x, newid;
     do {