netns: provide pure entropy for net_hash_mix()

net_hash_mix() currently uses kernel address of a struct net,
and is used in many places that could be used to reveal this
address to a patient attacker, thus defeating KASLR, for
the typical case (initial net namespace, &init_net is
not dynamically allocated)

I believe the original implementation tried to avoid spending
too many cycles in this function, but security comes first.

Also provide entropy regardless of CONFIG_NET_NS.

Fixes: 0b4419162aa6 ("netns: introduce the net_hash_mix "salt" for hashes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Amit Klein <aksecurity@gmail.com>
Reported-by: Benny Pinkas <benny@pinkas.net>
Cc: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
void unregister_pernet_device(struct pernet_operations *ops)
{
	down_write(&pernet_ops_rwsem);
	if (&ops->list == first_device)
		first_device = first_device->next;
	unregister_pernet_operations(ops);
	up_write(&pernet_ops_rwsem);
}

void unregister_pernet_device(struct pernet_operations *ops)
{
	down_write(&pernet_ops_rwsem);
	if (&ops->list == first_device)
		first_device = first_device->next;
	unregister_pernet_operations(ops);
	up_write(&pernet_ops_rwsem);
}
