QMessageAuthenticationCode: re-use messageHash for hashing the key

With the OpenSSL3 code allocating state on the heap instead of in
QCH::Private's inline union, reset() should be faster than even a
static hash() call. Even in the non-OpenSSL3 case, using less
QCH::Private objects to do the same thing means we increase effective
data cache size.

Pick-to: 6.5
Change-Id: I0b1347864081169a24c5d349702931afdab6c5bf
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qcryptographichash.cpp

@@ -1002,12 +1002,22 @@ public:
     // END functions that need to be called with finalizeMutex held
 };
 
+/*!
+    \internal
+
+    Transforms key into a block-sized format and then seeds messageHash from it.
+
+    This function assumes that messageHash is in its initial state (reset() has
+    been called).
+*/
 void QMessageAuthenticationCodePrivate::initMessageHash()
 {
     const int blockSize = qt_hash_block_size(method);
 
     if (key.size() > blockSize) {
-        key = QCryptographicHash::hash(key, method);
+        messageHash.addData(key);
+        key = messageHash.result();
+        messageHash.reset();
     }
 
     if (key.size() < blockSize) {