[PMTUd] Use kernel MTU information to determine next packet size during discovery

Using this information we can, for good links (*), determine and verify the link
MTU with 2 packets.

• good links means:

node X has MTU Y configured on a given interface. Any network object between node X
and destination is capable of handling MTU >= Y.
In no case the kernel will allow us to send packets > Y.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>