virtio_net: Remove command data from control_buf

Allocate memory for the data when it's used. Ideally the struct could
be on the stack, but we can't DMA stack memory. With this change only
the header and status memory are shared between commands, which will
allow using a tighter lock than RTNL.

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>