fix loop autoclear for xfstets xfs/049

[Christoph Hellwig <hch@xxxxxx>]

Hi Jens, hi Tetsuo,

this is a 3rd approach to fix the loop autoclean delay.  Instead of
working around the workqueue lockdep issues this switches the loop
driver to use a global workqueue and thus avoids the destroy_workqueue
call under disk->open_mutex entirely.