in6_lifaddr_ioctl SIOCDLIFADDR copies prefix mask into ifra_dstaddr instead of ifra_prefixmask

Summary

SIOCDLIFADDR(:1606) bcopy(&ia_prefixmask,&ifra.ifra_dstaddr,...) should be &ifra.ifra_prefixmask. ifra_prefixmask left zero, ifra_dstaddr clobbered with mask. Latent: recursive SIOCDIFADDR re-looks-up live ia, doesnt trust aliasreq fields. Future regression risk. Upstream uses prefixmask.