[DLM] fix grant_after_purge softlockup

In dlm_grant_after_purge() we were holding a hash table read_lock while
calling put_rsb() which potentially removes the rsb from the hash table,
taking the same lock in write.  Fix this by flagging rsb's ahead of time
that have been purged.  Then iteratively read_lock the hash table, find a
flagged rsb, unlock, process rsb.

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