Use -1 to identify the post-initial MTU discovery state.

Use -1 to identify the post-initial MTU discovery state.

This is a minor cosmetic nit to emphasise the distinction between the
initial MTU discovery phase, and the post-initial phase (i.e. maxmtu
checking).

Furthermore, this is an improvement with regard to the DRY (Don't
Repeat Yourself) principle, as the maximum mtuprobes value is only
written once.