can: rx-offload: can_rx_offload_offload_one(): increment rx_fifo_errors on queue overflow or OOM

If the rx-offload skb_queue is full or the skb allocation fails (due to OOM),
the mailbox contents is discarded.

This patch adds the incrementing of the rx_fifo_errors statistics counter.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/rx-offload.c

@@ -125,8 +125,10 @@ static struct sk_buff *can_rx_offload_offload_one(struct can_rx_offload *offload
 
 		ret = offload->mailbox_read(offload, &cf_overflow,
 					    &timestamp, n);
-		if (ret)
+		if (ret) {
 			offload->dev->stats.rx_dropped++;
+			offload->dev->stats.rx_fifo_errors++;
+		}
 
 		return NULL;
 	}