[DLM] keep dlm from panicing when traversing rsb list in debugfs

This problem was originally reported against GFS6.1, but the same issue exists
in upstream DLM.  This patch keeps the rsb iterator assigning under the rsbtbl
list lock.  Each time we process an rsb we grab a reference to it to make sure
it is not freed out from underneath us, and then put it when we get the next rsb
in the list or move onto another list.

Signed-off-by: Josef Bacik <jwhiter@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>