[DLM] status messages ping-pong between unmounted nodes

Red Hat BZ 213682

If two nodes leave the lockspace (while unmounting the fs in the case of
gfs) after one has sent a STATUS message to the other, STATUS/STATUS_REPLY
messages will then ping-pong between the nodes when neither of them can
find the lockspace in question any longer.  We kill this by not sending
another STATUS message when we get a STATUS_REPLY for an unknown
lockspace.

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