Recalculate and resend MTU probes if they are too large for the system.

Currently, if a MTU probe is sent and gets rejected by the system
because it is too large (i.e. send() returns EMSGSIZE), the MTU
discovery algorithm is not aware of it and still behaves as if the probe
was actually sent.

This patch makes the MTU discovery algorithm recalculate and send a new
probe when this happens, so that the probe "slot" does not go to waste.