sfc: detach from cb_page in efx_copy_channel()

It's a resource, not a parameter, so we can't copy it into the new
 channel's TX queues, otherwise aliasing will lead to resource-
 management bugs if the channel is subsequently torn down without
 being initialised.

Before the Fixes:-tagged commit there was a similar bug with
 tsoh_page, but I'm not sure it's worth doing another fix for such
 old kernels.

Fixes: e9117e50 ("sfc: Firmware-Assisted TSO version 2")
Suggested-by: Derek Shute <Derek.Shute@stratus.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/efx_channels.c

@@ -583,6 +583,7 @@ struct efx_channel *efx_copy_channel(const struct efx_channel *old_channel)
 		if (tx_queue->channel)
 			tx_queue->channel = channel;
 		tx_queue->buffer = NULL;
+		tx_queue->cb_page = NULL;
 		memset(&tx_queue->txd, 0, sizeof(tx_queue->txd));
 	}