[IPV6]: Ensure to truncate result and return full length for sticky options.

Bug noticed by Chris Wright <chrisw@sous-sol.org>.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -825,9 +825,9 @@ static int ipv6_getsockopt_sticky(struct sock *sk, struct ipv6_txoptions *opt,
 		return 0;
 
 	len = min_t(unsigned int, len, ipv6_optlen(hdr));
-	if (copy_to_user(optval, hdr, ipv6_optlen(hdr)))
+	if (copy_to_user(optval, hdr, len));
 		return -EFAULT;
-	return len;
+	return ipv6_optlen(hdr);
 }
 
 static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,