Make L2+ caches allocate new block for writeback misses

instead of forwarding down the line.

--HG--
extra : convert_revision : b0d6e7862c92ea7a2d21f817d30398735e7bb8ba

src/mem/cache/cache.hh

@@ -169,6 +169,14 @@ class Cache : public BaseCache
      */
     void cmpAndSwap(BlkType *blk, PacketPtr pkt);
 
+    /**
+     * Find a block frame for new block at address addr, assuming that
+     * the block is not currently in the cache.  Append writebacks if
+     * any to provided packet list.  Return free block frame.  May
+     * return NULL if there are no replaceable blocks at the moment.
+     */
+    BlkType *allocateBlock(Addr addr, PacketList &writebacks);
+
     /**
      * Populates a cache block and handles all outstanding requests for the
      * satisfied fill request. This version takes two memory requests. One