net: ipv6: mcast: minor: use defines for rfc3810/8.1 lengths

Instead of hard-coding length values, use a define to make it clear
where those lengths come from.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>