Author: Fabio M. Di Nitto <fdinitto(a)redhat.com>
Date: 2014-11-30 (Sun, 30 Nov 2014)
[PMTUd] distinguish between link_mtu and data_mtu
link_mtu is the pure MTU over the link and doesn't account for knet headers.
data_mtu is what can be really transmitted onwire by the application.
Signed-off-by: Fabio M. Di Nitto <fdinitto(a)redhat.com>