Add missing lock calls in isc__nm_tcpdns_stoplistening()

Closed Mark Andrews requested to merge marka-missing-lock-in-isc__nm_tcpdns_stoplistening into main

isc__nmsocket_clearcb() can update a socket being processed on a different thread and without the lock calls tsan errors can be produced between isc__nm_tcpdns_stoplistening() and processbuffer().

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