fix external memory request nack logic

rocket/src/main/scala/ctrl.scala

@@ -806,7 +806,7 @@ class rocketCtrl extends Component
   io.dmem.req_cmd     := ex_reg_mem_cmd;
   io.dmem.req_type    := ex_reg_mem_type;
 
-  io.ext_mem.resp_nack:= mem_reg_ext_mem_val && (io.dmem.resp_nack || Reg(!io.dmem.req_rdy))
+  io.ext_mem.resp_nack:= mem_reg_ext_mem_val && (io.dmem.req_kill || io.dmem.resp_nack || Reg(!io.dmem.req_rdy))
 }
 
 }