isisd: always join all IS-IS multicast groups

isisd: always join all IS-IS multicast groups

The socket is only created once when an interface is brought up, and the
multicast groups were joined according to configuration at that point.
This breaks when later switching an interface to another IS-IS level.

Since, for a separate conformance issue (ANVL ISIS-6.4), we should be
inspecting the destination address anyway, the simplest fix here is to
just join all groups unconditionally.  There shouldn't be much traffic
on these anyway, worst case we might be picking up some unrelated
multicast groups due to NIC filter aliasing though...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Tested-by: Martin Winter <mwinter@opensourcerouting.org>