[IPV6]: Fix the data length of get destination options with short length

If get destination options with length which is not enough for that
option,getsockopt() will still return the real length of the option,
which is larger then the buffer space.
 This is because ipv6_getsockopt_sticky() returns the real length of
the option.

This patch fix this problem.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/ipv6_sockglue.c

@@ -832,7 +832,7 @@ static int ipv6_getsockopt_sticky(struct sock *sk, struct ipv6_txoptions *opt,
 	len = min_t(unsigned int, len, ipv6_optlen(hdr));
 	if (copy_to_user(optval, hdr, len))
 		return -EFAULT;
-	return ipv6_optlen(hdr);
+	return len;
 }
 
 static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,