[IPV6]: Disallow RH0 by default (CVE-2007-2242)

A security issue is emerging.  Disallow Routing Header Type 0 by default
as we have been doing for IPv4.

This version already includes a fix for the original patch.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>