[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>