[TIPC]: Reduce stack usage

The node_map struct can be quite large (516 bytes) and allocating two of
them on the stack is not a good idea since we might only have a 4K stack
to start with.

Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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