mem: Add check for express snoop in packet destructor

Snoop packets share the request pointer with the originating
packets. We need to ensure that the snoop packet destruction does not
delete the request. Snoops are used for reads, invalidations,
HardPFReqs, Writebacks and CleansEvicts. Reads, invalidations, and
HardPFReqs need a response so their snoops do not delete the
request. For Writebacks and CleanEvicts we need to check explicitly
for whethere the current packet is an express snoop, in whcih case do
not delete the request.

src/mem/packet.hh

@@ -692,11 +692,18 @@ class Packet : public Printable
      */
     ~Packet()
     {
-        // If this is a request packet for which there's no response,
-        // delete the request object here, since the requester will
-        // never get the chance.
-        if (req && isRequest() && !needsResponse())
+        // Delete the request object if this is a request packet which
+        // does not need a response, because the requester will not get
+        // a chance. If the request packet needs a response then the
+        // request will be deleted on receipt of the response
+        // packet. We also make sure to never delete the request for
+        // express snoops, even for cases when responses are not
+        // needed (CleanEvict and Writeback), since the snoop packet
+        // re-uses the same request.
+        if (req && isRequest() && !needsResponse() &&
+            !isExpressSnoop()) {
             delete req;
+        }
         deleteData();
     }