CVE-2026-63996: ethtool: cmis: require exact CDB reply length

In the Linux kernel, the following vulnerability has been resolved:

ethtool: cmis: require exact CDB reply length

Malicious SFP module could respond with rpllen longer than what cmiscdbprocessreply() expected, leading to OOB writes. Malicious HW is a bit theoretical but some modules may just be buggy and/or the reads may occasionally get corrupted, so let's protect the kernel.

The existing check protects from short replies. We need to protect from long ones, too. All callers that pass a non-zero rplexplen cast the reply payload to a fixed-layout struct and read fields at fixed offsets, with no version negotiation or short-reply handling:

- cmiscdbvalidatepassword() - cmiscdbmodulefeaturesget() - cmisfwupdatefwmngfeaturesget()

so let's assume that responses longer than expected do not have to be handled gracefully here. Add a warning message to make the debug easier in case my understanding is wrong...

Note that pagedata->length (argument of kmalloc) comes from last arg to ethtoolcmispageinit() which is rplexplen.

Note2 that AIs also like to point out overflows in args->req.payload itself (which is a fixed-size 120 B buffer, on the stack), but callers should be reading structs defined by the standard, so protecting from requests for more data than max seem like defensive programming.