mpmd: Add MTU plausibility check

We have noticed that on 1 GbE connections, MTU discovery can become
unreliable. Since we now use the MTU directly for deriving spp and other
values, a correct MTU is important.

Because we don't have a way of knowing if MTU discovery worked or not,
we add some heuristics in form of a plausibility check. For now, the
only rule in this check is if that the detected MTU is a bit larger than
1472 bytes, we coerce down to 1472, because this is such a standard
value (most 1 GbE interfaces default to an IPv4 MTU of 1500 bytes).

For the cases where the interface MTU is set to be between 1500 and 1528
bytes, this would cause a very minor performance loss. We accept this
performance loss as it is small, and those cases are very rare. MTUs are
usually 1500 bytes, or >= 8000 bytes for high-speed links using jumbo
frames.

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