[DLM] set purged flag on rsbs

If a node becomes the new master of an rsb during recovery, the
LOCKS_PURGED flag needs to be set on it so that any waiting/converting
locks will try to be granted.

Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>