powerpc/kasan: Silence KASAN warnings in _getwchan()
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.4.200 by commit 348831a9e8aa311e6fb09ce8748b8ed29e73cf05. For more details please see the references link.