tipc: purge links when bearer is disabled

If a bearer is disabled by manual intervention, all links over that
bearer should be purged, indicated with the 'shutting_down' flag.
Otherwise tipc will get confused if a new bearer is enabled using
a different media type.

Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>