use of the system work queue in ublk

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

Hi Ming,

it seems like ublk uses schedule_work to stop the device, which
includes a del_gendisk.  I'm a little fearful this will gets us into
lockdep chains of death once syzbot or Tetsu notice it.

That being said, I don't reall understand the design of
ublk_daemon_monitor_work, which is only used to kick off other
work to start with.