fs: dlm: debugfs for queued callbacks

It was useful to debug an issue with the callback queue to check if any
callbacks in any lkb are for some reason not processed by the callback
workqueue. The mentioned issue was fixed by commit a034c137 ("fs:
dlm: fix DLM_IFL_CB_PENDING gets overwritten"). If there are similar
issue that looks like a ast callback was not processed, we can confirm
now that it is not sitting to be processed by the callback workqueue
anymore.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>