mem-cache: Add stats for failed compressions

mem-cache: Add stats for failed compressions

Add statistics to keep track of the number of times compression
has failed to provide blocks whose compressed size passes the
size threshold.

Also, update the compressed data's size if compression fails.

Change-Id: If3479572bf114f07911238c602ffef3a90b6a931
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33384
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>