NGM_FEC_SET_MODE_INET6 sets mode unhandled by output path: all packets silently dropped

Summary

ng_fec_rcvmsg accepts NGM_FEC_SET_MODE_INET6 sets b->fec_btype=FEC_BTYPE_INET6(:1188-1189). ng_fec_output_serialized switch only handles FEC_BTYPE_MAC and FEC_BTYPE_INET(:810-833). INET6 falls to default: bogus hash type + m_freem(:834-839). Every frame dropped. INET6 cmd nested inside #ifdef INET so no-INET+INET6 build omits command. NOT same as DF-0507(ng7 IPv6 hash XOR self): v1 correctly computes src^dst. Fix: add FEC_BTYPE_INET6 case, de-nest from #ifdef INET.