in6_update_ifa inconsistent error handling on multicast group joins

Summary

Solicited-node join failure fatal: in6_purgeaddr+return(:1118-1127). All-nodes/node-info join failure: LOG_WARNING+continue(:1151-1175,:1200-1207). Address installed+routed but missing mandatory ff02::1 membership -> broken ND/MLD. Transient ENOMEM trigger.