mcastmngr: pass max groups parameter to mcproxy
Setting '/proc/sys/net/ipv4/igmp_max_memberships' is not enough. We must pass the max_groups parameter to the mcproxy to prevent MFC entries from being added to mr_table and packet forwarding for extra groups.
We also need to increase max_groups, since mcproxy always subscribe two service groups - ALL-ROUTERS (224.0.0.2) and ALL-IGMPv3-MC-ROUTERS (224.0.0.22).