usbnet: Fix tx_bytes statistic running backward in cdc_ncm

cdc_ncm disagrees with usbnet about how much framing overhead should
be counted in the tx_bytes statistics, and tries 'fix' this by
decrementing tx_bytes on the transmit path.  But statistics must never
be decremented except due to roll-over; this will thoroughly confuse
user-space.  Also, tx_bytes is only incremented by usbnet in the
completion path.

Fix this by requiring drivers that set FLAG_MULTI_FRAME to set a
tx_bytes delta along with the tx_packets count.

Fixes: beeecd42c3 ("net: cdc_ncm/cdc_mbim: adding NCM protocol statistics")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Bjørn Mork <bjorn@mork.no>

drivers/net/usb/cdc_ncm.c

@@ -1177,13 +1177,12 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
 	ctx->tx_overhead += skb_out->len - ctx->tx_curr_frame_payload;
 	ctx->tx_ntbs++;
 
-	/* usbnet has already counted all the framing overhead.
+	/* usbnet will count all the framing overhead by default.
 	 * Adjust the stats so that the tx_bytes counter show real
 	 * payload data instead.
 	 */
-	dev->net->stats.tx_bytes -= skb_out->len - ctx->tx_curr_frame_payload;
-
-	usbnet_set_skb_tx_stats(skb_out, n);
+	usbnet_set_skb_tx_stats(skb_out, n,
+				ctx->tx_curr_frame_payload - skb_out->len);
 
 	return skb_out;