gve: prevent ethtool ops after shutdown

In the Linux kernel, the following vulnerability has been resolved: gve: prevent ethtool ops after shutdown A crash can occur if an ethtool operation is invoked after shutdown() is called. shutdown() is invoked during system shutdown to stop DMA operations without performing expensive deallocations. It is discouraged to unregister the netdev in this path, so the device may still be visible to userspace and kernel helpers. In gve, shutdown() tears down most internal data structures. If an ethtool operation is dispatched after shutdown(), it will dereference freed or NULL pointers, leading to a kernel panic. While graceful shutdown normally quiesces userspace before invoking the reboot syscall, forced shutdowns (as observed on GCP VMs) can still trigger this path. Fix by calling netif_device_detach() in shutdown(). This marks the device as detached so the ethtool ioctl handler will skip dispatching operations to the driver.