Use mountFd on darwin too.

fuse/mountstate_darwin.go

 package fuse
 
+import (
+	"syscall"
+)
+
 func (ms *MountState) systemWrite(req *request, header []byte) Status {
 	if req.flatDataSize() == 0 {
-		_, err := ms.mountFile.Write(header)
+		_, err := syscall.Write(ms.mountFd, Write(header))
 		return ToStatus(err)
 	}
 
@@ -13,7 +17,7 @@ func (ms *MountState) systemWrite(req *request, header []byte) Status {
 		header = req.serializeHeader(len(req.flatData))
 	}
 
-	_, err := Writev(int(ms.mountFile.Fd()), [][]byte{header, req.flatData})
+	_, err := Writev(int(ms.mountFd), [][]byte{header, req.flatData})
 	if req.readResult != nil {
 		req.readResult.Done()
 	}