mem-cache: Copy over flags to forwarded response

mem-cache: Copy over flags to forwarded response

A cache that forwards a request to the memory below does not fill and
forwards the response with the data to cache above. This change
ensures that the flags of the original response are also preserved.

Change-Id: I244b20b073c31b976358816c5b14bba413b8271f
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16182
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>