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 __inline__ int icmp_filter(struct sock *sk, struct sk_buff *skb)
{
	int type;

	if (!pskb_may_pull(skb, sizeof(struct icmphdr)))
		return 1;

	type = icmp_hdr(skb)->type;
	if (type < 32) {
		__u32 data = raw_sk(sk)->filter.data;

		return ((1 << type) & data) != 0;
	}

	/* Do not block unknown ICMP types */
	return 0;
}

static __inline__ int icmp_filter(struct sock *sk, struct sk_buff *skb)
{
	int type;

	if (!pskb_may_pull(skb, sizeof(struct icmphdr)))
		return 1;

	type = icmp_hdr(skb)->type;
	if (type < 32) {
		__u32 data = raw_sk(sk)->filter.data;

		return ((1 << type) & data) != 0;
	}

	/* Do not block unknown ICMP types */
	return 0;
}
