tipc: simplify handling of lookup scope during multicast message reception

tipc: simplify handling of lookup scope during multicast message reception

We introduce a new macro TIPC_ANY_SCOPE to make the handling of the
lookup scope value more comprehensible during multicast reception.

The (unchanged) rules go as follows:

1) Multicast messages sent from own node are delivered to all matching
   sockets on the own node, irrespective of their binding scope.

2) Multicast messages sent from other nodes arrive here because they
   have found TIPC_CLUSTER_SCOPE bindings emanating from this node.
   Those messages should be delivered to exactly those sockets, but not
   to local sockets bound with TIPC_NODE_SCOPE, since the latter
   obviously were not meant to be visible for those senders.

3) Group multicast/broadcast messages are delivered to the sockets with
   a binding scope matching exactly the lookup scope indicated in the
   message header, and nobody else.

Reviewed-by: Xin Long <lucien.xin@gmail.com>
Tested-by: Hoang Le <hoang.h.le@dektech.com.au>
Signed-off-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>