scsi: virtio_scsi: don't send sc payload with tmfs

The virtio scsi spec defines struct virtio_scsi_ctrl_tmf as a set of
device-readable records and a single device-writable response entry:

    struct virtio_scsi_ctrl_tmf
    {
        // Device-readable part
        le32 type;
        le32 subtype;
        u8 lun[8];
        le64 id;
        // Device-writable part
        u8 response;
    }

The above should be organised as two descriptor entries (or potentially
more if using VIRTIO_F_ANY_LAYOUT), but without any extra data after "le64
id" or after "u8 response".

The Linux driver doesn't respect that, with virtscsi_abort() and
virtscsi_device_reset() setting cmd->sc before calling virtscsi_tmf().  It
results in the original scsi command payload (or writable buffers) added to
the tmf.

This fixes the problem by leaving cmd->sc zeroed out, which makes
virtscsi_kick_cmd() add the tmf to the control vq without any payload.

Cc: stable@vger.kernel.org
Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>