route: let route/link join RTNLGRP_IPV6_IFINFO mcast group
route: let route/link join RTNLGRP_IPV6_IFINFO mcast group

Required to be notified about inet6 managed/otherconf flag changes, see
occurrences of inet6_ifinfo_notify in net/ipv6/ndisc.c of the Linux
kernel.

https://github.com/thom311/libnl/pull/264

thom311/libnl,thom311/libnl,greearb/libnl-ct,thom311/libnl,greearb/libnl-ct,greearb/libnl-ct