dlm: fix seq_file usage in debugfs lock dump

The old code would leak iterators and leave reference counts on
rsbs because it was ignoring the "stop" seq callback.  The code
followed an example that used the seq operations differently.
This new code is based on actually understanding how the seq
operations work.  It also improves things by saving the hash bucket
in the position to avoid cycling through completed buckets in start.

Siged-off-by: Davd Teigland <teigland@redhat.com>