Ensure mremap boundaries not trusting the guest kernel to
pass the correct buffer length.

Fixes: CVE-2021-3582
Reported-by: VictorV (Kunlun Lab) <vv474172261@gmail.com>
Tested-by: VictorV (Kunlun Lab) <vv474172261@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Message-Id: <20210616110600.20889-1-marcel.apfelbaum@gmail.com>
Reviewed-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Tested-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>