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>
static int rtnl_net_fill(struct sk_buff *skb, struct net_fill_args *args)
{
	struct nlmsghdr *nlh;
	struct rtgenmsg *rth;

	nlh = nlmsg_put(skb, args->portid, args->seq, args->cmd, sizeof(*rth),
			args->flags);
	if (!nlh)
		return -EMSGSIZE;

	rth = nlmsg_data(nlh);
	rth->rtgen_family = AF_UNSPEC;

	if (nla_put_s32(skb, NETNSA_NSID, args->nsid))
		goto nla_put_failure;

	if (args->add_ref &&
	    nla_put_s32(skb, NETNSA_CURRENT_NSID, args->ref_nsid))
		goto nla_put_failure;

	nlmsg_end(skb, nlh);
	return 0;

nla_put_failure:
	nlmsg_cancel(skb, nlh);
	return -EMSGSIZE;
}

static int rtnl_net_fill(struct sk_buff *skb, struct net_fill_args *args)
{
	struct nlmsghdr *nlh;
	struct rtgenmsg *rth;

	nlh = nlmsg_put(skb, args->portid, args->seq, args->cmd, sizeof(*rth),
			args->flags);
	if (!nlh)
		return -EMSGSIZE;

	rth = nlmsg_data(nlh);
	rth->rtgen_family = AF_UNSPEC;

	if (nla_put_s32(skb, NETNSA_NSID, args->nsid))
		goto nla_put_failure;

	if (args->add_ref &&
	    nla_put_s32(skb, NETNSA_CURRENT_NSID, args->ref_nsid))
		goto nla_put_failure;

	nlmsg_end(skb, nlh);
	return 0;

nla_put_failure:
	nlmsg_cancel(skb, nlh);
	return -EMSGSIZE;
}
