mailmap: Update Oliver's email address

While I'm still at Google, I've since switched to a linux.dev account
for working upstream.

Add an alias to the new address.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Message-Id: <20220819190158.234290-1-oliver.upton@linux.dev>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>