fuse: don't write INTERRUPT reply

INTERRUPT does not require a reply to the kernel. Trying to send it
yields

  writer: Write/Writev failed, err: 2=no such file or
  directory. opcode: INTERRUPT

fuse/opcode.go

@@ -457,6 +457,9 @@ func doSetLkw(server *Server, req *request) {
 	req.status = server.fileSystem.SetLkw((*LkIn)(req.inData))
 }
 
+func doInterrupt(server *Server, req *request) {
+}
+
 ////////////////////////////////////////////////////////////////
 
 type operationFunc func(*Server, *request)
@@ -664,6 +667,7 @@ func init() {
 		_OP_FALLOCATE:    doFallocate,
 		_OP_READDIRPLUS:  doReadDirPlus,
 		_OP_RENAME2:      doRename2,
+		_OP_INTERRUPT:    doInterrupt,
 	} {
 		operationHandlers[op].Func = v
 	}

fuse/server.go

@@ -455,7 +455,7 @@ func (ms *Server) allocOut(req *request, size uint32) []byte {
 func (ms *Server) write(req *request) Status {
 	// Forget/NotifyReply do not wait for reply from filesystem server.
 	switch req.inHeader.Opcode {
-	case _OP_FORGET, _OP_BATCH_FORGET, _OP_NOTIFY_REPLY:
+	case _OP_FORGET, _OP_BATCH_FORGET, _OP_NOTIFY_REPLY, _OP_INTERRUPT:
 		return OK
 	}