mem: Do not allocate space for packet data if not needed

mem: Do not allocate space for packet data if not needed

This patch looks at the request and response command to determine if
either actually has any data payload, and if not, we do not allocate
any space for packet data.

The only tricky case is where the command type is changed as part of
the MSHR functionality. In these cases where the original packet had
no data, but the new packet does, we need to explicitly call
allocate().