dlm: prevent connections during shutdown

During lowcomms shutdown, a new connection could possibly
be created, and attempt to use a workqueue that's been
destroyed.  Similarly, during startup, a new connection
could attempt to use a workqueue that's not been set up
yet.  Add a global variable to indicate when new connections
are allowed.

Based on patch by: Christine Caulfield <ccaulfie@redhat.com>

Reported-by: dann frazier <dann.frazier@canonical.com>
Reviewed-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: David Teigland <teigland@redhat.com>