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 void raw_sock_seq_show(struct seq_file *seq, struct sock *sp, int i)
{
	struct inet_sock *inet = inet_sk(sp);
	__be32 dest = inet->inet_daddr,
	       src = inet->inet_rcv_saddr;
	__u16 destp = 0,
	      srcp  = inet->inet_num;

	seq_printf(seq, "%4d: %08X:%04X %08X:%04X"
		" %02X %08X:%08X %02X:%08lX %08X %5d %8d %lu %d %p %d\n",
		i, src, srcp, dest, destp, sp->sk_state,
		sk_wmem_alloc_get(sp),
		sk_rmem_alloc_get(sp),
		0, 0L, 0, sock_i_uid(sp), 0, sock_i_ino(sp),
		atomic_read(&sp->sk_refcnt), sp, atomic_read(&sp->sk_drops));
}

static void raw_sock_seq_show(struct seq_file *seq, struct sock *sp, int i)
{
	struct inet_sock *inet = inet_sk(sp);
	__be32 dest = inet->inet_daddr,
	       src = inet->inet_rcv_saddr;
	__u16 destp = 0,
	      srcp  = inet->inet_num;

	seq_printf(seq, "%4d: %08X:%04X %08X:%04X"
		" %02X %08X:%08X %02X:%08lX %08X %5d %8d %lu %d %p %d\n",
		i, src, srcp, dest, destp, sp->sk_state,
		sk_wmem_alloc_get(sp),
		sk_rmem_alloc_get(sp),
		0, 0L, 0, sock_i_uid(sp), 0, sock_i_ino(sp),
		atomic_read(&sp->sk_refcnt), sp, atomic_read(&sp->sk_drops));
}
