virtio_net: Fix error handling in virtnet_restore()

Do some cleanups in virtnet_restore() when virtnet_cpu_notif_add() failed.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210517084516.332-1-xieyongji@bytedance.com
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>