virtio_net: Fix panic in virtnet_remove

Fix a panic in virtnet_remove. unregister_netdev has already
freed up the netdev (and virtnet_info) due to dev->destructor
being set, while virtnet_info is still required. Remove
virtnet_free altogether, and move the freeing of the per-cpu
statistics from virtnet_free to virtnet_remove.

Tested patch below.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>