virtio_pci_modern: treat vp_dev->admin_vq.info.vq pointer as static

It is guaranteed by the virtio_pci and PCI layers that none of the VFs
is probed before setup_vq() is called for admin queue and after del_vq()
is called for admin queue. Therefore treat vp_dev->admin_vq.info.vq as
static, don't null it and don't take cmd lock during assign.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-7-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>