[NetKVM] BZ#950633 Checksum verification fixed
[NetKVM] BZ#950633 Checksum verification fixed

Checksums 0xFFFF and 0x0000 treated equal
as suggested by RFC 1624 sec. 3

GoogleCloudPlatform/compute-windows-drivers,virtio-win/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,YanVugenfirer/virtio-win-arm,YanVugenfirer/virtio-win-arm,YanVugenfirer/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,virtio-win/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,gnif/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,YanVugenfirer/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,vrozenfe/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows