DiscIO: Store partition key directly in ReuseID

This lets us reuse blocks in the case where different partition
entries use the same key (which probably isn't very common).

Source/Core/DiscIO/WIABlob.h

@@ -258,7 +258,7 @@ private:
 
 #undef COMPARE_TIED
 
-    const WiiKey* partition_key;
+    WiiKey partition_key;
     u64 data_size;
     bool encrypted;
     u8 value;