Local kernel compromise via duplicate netdev hook registration

Use-after-free in Linux Kernel 6.16 through 6.18.18 and 6.19 through 6.19.8 allows local users to crash the kernel. NETDEV_REGISTER handling in nft_flowtable_event and nft_netdev_event adds a second hook when an existing device hook is already present, leaving stale hook state that dereferences the freed net_device during teardown. Exploitation requires CAP_NET_ADMIN in the target network namespace; on deployments exposing user and network namespaces, an unprivileged local user can reach the path and corrupt kernel state or disclose kernel memory.