ipv6: add complete rcu protection around np->opt

This patch addresses multiple problems :

UDP/RAW sendmsg() need to get a stable struct ipv6_txoptions
while socket is not locked : Other threads can change np->opt
concurrently. Dmitry posted a syzkaller
(http://github.com/google/syzkaller) program desmonstrating
use-after-free.

Starting with TCP/DCCP lockless listeners, tcp_v6_syn_recv_sock()
and dccp_v6_request_recv_sock() also need to use RCU protection
to dereference np->opt once (before calling ipv6_dup_options())

This patch adds full RCU protection to np->opt

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
int ip6_datagram_connect_v6_only(struct sock *sk, struct sockaddr *uaddr,
				 int addr_len)
{
	DECLARE_SOCKADDR(struct sockaddr_in6 *, sin6, uaddr);
	if (sin6->sin6_family != AF_INET6)
		return -EAFNOSUPPORT;
	return ip6_datagram_connect(sk, uaddr, addr_len);
}

int ip6_datagram_connect_v6_only(struct sock *sk, struct sockaddr *uaddr,
				 int addr_len)
{
	DECLARE_SOCKADDR(struct sockaddr_in6 *, sin6, uaddr);
	if (sin6->sin6_family != AF_INET6)
		return -EAFNOSUPPORT;
	return ip6_datagram_connect(sk, uaddr, addr_len);
}
