tipc: add net dump to new netlink api

Add TIPC_NL_NET_GET command to the new tipc netlink API.

This command dumps the network id of the node.

Netlink logical layout of returned network data:
-> net
    -> id

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>