The failover of fuse userspace daemon will reuse the existing fuse conn,
without unmounting it, during daemon crashing and recovery procedure.
But some requests might be in process in the daemon before sending out reply,
when the crash happens. This will stuck the application since it will
never get the reply after the failover.

We add the sysfs api to flush these requests, after the daemon crash, before
recovery. It is easy to reproduce the issue in the fuse userspace daemon,
just exit after receiving the request and before sending the reply back.
The application will hang up in some read/write operation, before
echo 1 > /sys/fs/fuse/connection/xxx/flush. The flush operation will make
the io fail and return the error to the application.
Signed-off-by: NMa Jie Yue <majieyue@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>