Ignore IPv6 routing cache on linux

Recent linux kernels (roughly 3.x) keep a routing cache for IPv6: each
time a host is contacted, a temporary /128 route is added. These
routes are visible using “ip -6 route show cache”.

Asking babeld to redistribute all IPv6 routes known to the host, using
"redistribute ip ::/0", causes it to announce and de-announce these
/128 routes frequently, leading other nodes to mark them as
unreachable (thus preventing communication with the host described by
the /128 route, even though less specific routes are available).