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: e9117e5099ea ("sfc: Firmware-Assisted TSO version 2")
Suggested-by: Derek Shute <[e-mail]>
Signed-off-by: Edward Cree <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/sfc/efx_channels.c

···
 		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));
 	}