tipc: Detect duplicate nodes using different network interfaces

Utilizes the new "node signature" field in neighbor discovery messages
to ensure that all links TIPC associates with a given <Z.C.N> network
address belong to the same neighboring node. (Previously, TIPC could not
tell if link setup requests arriving on different interfaces were from
the same node or from two different nodes that has mistakenly been assigned
the same network address.)

The revised algorithm for detecting a duplicate node considers both the
node signature and the network interface adddress specified in a request
message when deciding how to respond to a link setup request. This prevents
false alarms that might otherwise arise during normal network operation
under the following scenarios:

a) A neighboring node reboots. (The node's signature changes, but the
network interface address remains unchanged.)

b) A neighboring node's network interface is replaced. (The node's signature
remains unchanged, but the network interface address changes.)

c) A neighboring node is completely replaced. (The node's signature and
network interface address both change.)

The algorithm also handles cases in which a node reboots and re-establishes
its links to TIPC (or begins re-establishing those links) before TIPC
detects that it is using a new node signature. In such cases of "delayed
rediscovery" TIPC simply accepts the new signature without disrupting
communication that is already underway over the links.

Thanks to Laser [gotolaser@gmail.com] for his contributions to the
development of this enhancement.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

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