[IPV6]: Check length of optval provided by user in setsockopt().

Check length of setsockopt's optval, which provided by user, before copy it
from user space.
For POSIX compliant, return -EINVAL for setsockopt of short lengths.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>