dlm: Sanity check namelen before copying it

The 32/64 compatibility code in the DLM does not check the validity of the lock name length passed into it, so it can easily overwrite memory if the value is rubbish (as early versions of libdlm can cause with unlock calls, it doesn't zero the field).

This patch restricts the length of the name to the amount of data actually passed into the call.

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