ipv6: Check the hop limit setting in ancillary data.

When specifing the outgoing hop limit as ancillary data for sendmsg(),
the kernel doesn't check the integer hop limit value as specified in
[RFC-3542] section 6.3.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>