defaultAndValidateMachineSpec() only validates, doesn't default

Unless I'm very confused right now, this code in defaultAndValidateMachineSpec() (part the of the mutating webhooks)

https://github.com/kubermatic/machine-controller/blob/94e2005cbd847a05e72d04ce97dc2d3bab59fa1c/pkg/admission/machines.go#L185

...only updates the passed-in pointer to the to-be-defaulted MachineSpec, but doesn't update the MachineSpec it points to. So the updated object will be garbage collected and the caller's MachineSpec will not be updated (i.e. defaulted).
Changing the line to something like `*spec = defaultedSpec` should fix it (or changing some function signatures so we pass pointers rather than values). I'm not sure about the implications because it seems like this bug has been in there since 2018. So maybe nobody really needed the provider-specific defaulting functionality?

Start in pkg/admission/machines.go around defaultAndValidateMachineSpec() at the linked line, then trace how its MachineSpec pointer is used by the mutating webhook caller. Confirm whether the caller's MachineSpec remains unchanged after defaulting; done means provider-specific defaults are present on the returned or mutated object and the existing behavior is covered by an appropriate regression test.

Written by the indexing model from the issue text.