mem: Add default initializers to the fields in Request.

mem: Add default initializers to the fields in Request.

This avoids having to have bunches of uninteresting initializers in the
Request constructors, and accidentally forgetting to initialize any of
them.

Change-Id: If7a91fdf4aa6cd774f6f53474f55034ed6eda5f0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26227
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>