[PMTUd] invalidate MTU for a link if the value is lower than minimum

Under heavy network load and packet loss, calculated MTU can be
too small. In that case we need to invalidate the link mtu,
that would remove the link from the rotation (and traffic) and
would give PMTUd time to get the right MTU in the next round.

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