Replace call to AllocatedBufferSize(), with BufferSize().
Indeed (quoting https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/portcls/nf-portcls-idmachannel-buffersize ):
> The BufferSize() method returns the buffer size that was set by the previous call to IDmaChannel::SetBufferSize(). If SetBufferSize() has not been called since the IDmaChannel::AllocateBuffer() call, BufferSize returns the allocated buffer size. The DMA-channel object does not actually use this value internally. This value is maintained by the object to **allow its various clients to communicate the intended size of the buffer**.
And this is exactly what we want to do.
Fixes a regression from 99fa38809f, which
replaced the kcom.h/stdunk.h versions (which zero memory) with local
implementations (which don't).
Standard C++ does not guarantee that memory is zeroed but several classes
rely on this, in particular for their m_Ref and m_bInitialized members.
Ideally the constructors should be fixed to initialize all required members,
but that task is error-prone so for now we simply restore the previous
behavior.
