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 sock *raw_get_next(struct seq_file *seq, struct sock *sk)
{
	struct raw_iter_state *state = raw_seq_private(seq);

	do {
		sk = sk_next(sk);
try_again:
		;
	} while (sk && sock_net(sk) != seq_file_net(seq));

	if (!sk && ++state->bucket < RAW_HTABLE_SIZE) {
		sk = sk_head(&state->h->ht[state->bucket]);
		goto try_again;
	}
	return sk;
}

static struct sock *raw_get_next(struct seq_file *seq, struct sock *sk)
{
	struct raw_iter_state *state = raw_seq_private(seq);

	do {
		sk = sk_next(sk);
try_again:
		;
	} while (sk && sock_net(sk) != seq_file_net(seq));

	if (!sk && ++state->bucket < RAW_HTABLE_SIZE) {
		sk = sk_head(&state->h->ht[state->bucket]);
		goto try_again;
	}
	return sk;
}
