[IPV6]: Fix the return value of get destination options with NULL data pointer

If we pass NULL data buffer to getsockopt(), it will return 0,
and the option length is set to -EFAULT:
    getsockopt(sk, IPPROTO_IPV6, IPV6_DSTOPTS, NULL, &len);

This is because ipv6_getsockopt_sticky() will return -EFAULT or
-EINVAL if some error occur.

This patch fix this problem.

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