tipc: increase size of tipc discovery messages

The payload area following the TIPC discovery message header is an
opaque area defined by the media. INT_H_SIZE was enough for
Ethernet/IB/IPv4 but needs to be expanded to carry IPv6 addressing
information.

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