inet: add RCU protection to inet->opt

We lack proper synchronization to manipulate inet->opt ip_options

Problem is ip_make_skb() calls ip_setup_cork() and
ip_setup_cork() possibly makes a copy of ipc->opt (struct ip_options),
without any protection against another thread manipulating inet->opt.

Another thread can change inet->opt pointer and free old one under us.

Use RCU to protect inet->opt (changed to inet->inet_opt).

Instead of handling atomic refcounts, just copy ip_options when
necessary, to avoid cache line dirtying.

We cant insert an rcu_head in struct ip_options since its included in
skb->cb[], so this patch is large because I had to introduce a new
ip_options_rcu structure.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
static struct inet_peer *tcp_v6_get_peer(struct sock *sk, bool *release_it)
{
	struct rt6_info *rt = (struct rt6_info *) __sk_dst_get(sk);
	struct ipv6_pinfo *np = inet6_sk(sk);
	struct inet_peer *peer;

	if (!rt ||
	    !ipv6_addr_equal(&np->daddr, &rt->rt6i_dst.addr)) {
		peer = inet_getpeer_v6(&np->daddr, 1);
		*release_it = true;
	} else {
		if (!rt->rt6i_peer)
			rt6_bind_peer(rt, 1);
		peer = rt->rt6i_peer;
		*release_it = false;
	}

	return peer;
}

static struct inet_peer *tcp_v6_get_peer(struct sock *sk, bool *release_it)
{
	struct rt6_info *rt = (struct rt6_info *) __sk_dst_get(sk);
	struct ipv6_pinfo *np = inet6_sk(sk);
	struct inet_peer *peer;

	if (!rt ||
	    !ipv6_addr_equal(&np->daddr, &rt->rt6i_dst.addr)) {
		peer = inet_getpeer_v6(&np->daddr, 1);
		*release_it = true;
	} else {
		if (!rt->rt6i_peer)
			rt6_bind_peer(rt, 1);
		peer = rt->rt6i_peer;
		*release_it = false;
	}

	return peer;
}
