This is not compatible with version 0.  Uses 24-byte messages and
doesn't distinguish routes from xroutes.

neighbour_rxcost() calls update_neighbour which might send some messages,
so make sure we don't call it in the middle of the txcost message.

This avoids inconsistent routing if a neighbour is ingress filtering a
route.

When one tries to add such a route, Linux will reply with EEXIST.

Thanks to Julien Cristau.