QObject: fix allocation of IDs for the undocumented userData() feature

This appears to have come to some fame on the internet, so better fix
the implementation before we remove it in Qt 6.

Change-Id: Ia37ca89105b13bea1ffcdce8b2e8cd957b7bd108
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qobject.cpp

@@ -4187,13 +4187,14 @@ void QObject::dumpObjectInfo() const
 }
 
 #ifndef QT_NO_USERDATA
+static QBasicAtomicInteger<uint> user_data_registration = Q_BASIC_ATOMIC_INITIALIZER(0);
+
 /*!
     \internal
  */
 uint QObject::registerUserData()
 {
-    static int user_data_registration = 0;
-    return user_data_registration++;
+    return user_data_registration.fetchAndAddRelaxed(1);
 }
 
 /*!