message: fuse: no need to check output param `exit` anymore
  If fuse service threads receive exit event, it just return Ok(None),
  so we still can jump out from that loop.
diff: @@ -159,11 +159,7 @@ impl FuseChannel {
  })
  }
  - pub fn get_reader<'b>(
  - &self,
  - buf: &'b mut Vec<u8>,
  - exit: &mut bool,
  - ) -> io::Result<Option<Reader<'b>>> {
  + pub fn get_reader<'b>(&self, buf: &'b mut Vec<u8>) -> io::Result<Option<Reader<'b>>> {
  loop {
  let num_events = epoll::wait(self.epoll_fd, -1, &mut self.events.borrow_mut())?;
  @@ -185,7 +181,6 @@ impl FuseChannel {
  // One more trick is we don't read the event fd so as to make all fuse threads exit.
  // That is because we configure this event fd as LEVEL triggered.
  info!("Will exit from fuse service");
  - *exit = true;
  return Ok(None);
  }
  