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.
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