destroy_workqueue() warnings still, at a lower frequency, trigger
spuriously.  The problem seems to be in-flight operations which
haven't reached put_pwq() yet.

* Make sanity check grab all the related locks so that it's
  synchronized against operations which puts pwq at the end.

* Always print out the offending pwq.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Williams, Gerald S" <gerald.s.williams@intel.com>