virtio_net: Remove rtnl lock protection of command buffers

The rtnl lock is no longer needed to protect the control buffer and
command VQ.

Signed-off-by: Daniel Jurgens <danielj@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Heng Qi <hengqi@linux.alibaba.com>
Tested-by: Heng Qi <hengqi@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>