netdev: prevent accessing NAPI instances from another namespace

In the Linux kernel, the following vulnerability has been resolved: netdev: prevent accessing NAPI instances from another namespace The NAPI IDs were not fully exposed to user space prior to the netlink API, so they were never namespaced. The netlink API must ensure that at the very least NAPI instance belongs to the same netns as the owner of the genl sock. napi_by_id() can become static now, but it needs to move because of dev_get_by_napi_id().