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().