manager: be stricter with incomining notifications, warn properly about too large ones

Let's make the kernel let us know the full, original datagram size of the
incoming message. If it's larger than the buffer space provided by us, drop the
whole message with a warning.

Before this change the kernel would truncate the message for us to the buffer
space provided, and we'd not complain about this, and simply process the
incomplete message as far as it made sense.