hv_netvsc: protect module refcount by checking net_device_ctx->vf_netdev

We're not guaranteed to see NETDEV_REGISTER/NETDEV_UNREGISTER notifications
only once per VF but we increase/decrease module refcount unconditionally.
Check vf_netdev to make sure we don't take/release it twice. We presume
that only one VF per netvsc device may exist.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>