ip6_mrouter_set performs no explicit capability check: relies entirely on raw-socket attach privilege

Summary

ip6_mrouter_set(:265-271) gates only on so!=ip6_mrouter&&name!=MRT6_INIT -> EACCES. Never calls caps_priv_check/priv_check. Indirectly gated by raw IPv6 socket (rip6_attach SYSCAP_NONET_RAW). Every other netinet6 control path does explicit caps_priv_check. Defense-in-depth gap; risk if raw-socket check loosened.