Because keventd is a resource which is shared between unrelated parts of the
kernel it is possible for one person's workqueue handler to accidentally call
another person's flush_scheduled_work().  thockin managed it by calling
mntput() from a workqueue handler.  It deadlocks.

It's simple enough to fix: teach flush_scheduled_work() to go direct when it
discovers that the calling thread is the one which should be running the
work.

Note that this can cause recursion.  The depth of that recursion is equal to
the number of currently-queued works which themselves want to call
flush_scheduled_work().  If this ever exceeds three I'll eat my hat.