can: flexcan: flexcan_mailbox_read() fix return value for drop = true

The following happened on an i.MX25 using flexcan with many packets on
the bus:

The rx-offload queue reached a length more than skb_queue_len_max. In
can_rx_offload_offload_one() the drop variable was set to true which
made the call to .mailbox_read() (here: flexcan_mailbox_read()) to
_always_ return ERR_PTR(-ENOBUFS) and drop the rx'ed CAN frame. So
can_rx_offload_offload_one() returned ERR_PTR(-ENOBUFS), too.

can_rx_offload_irq_offload_fifo() looks as follows:

| 	while (1) {
| 		skb = can_rx_offload_offload_one(offload, 0);
| 		if (IS_ERR(skb))
| 			continue;
| 		if (!skb)
| 			break;
| 		...
| 	}

The flexcan driver wrongly always returns ERR_PTR(-ENOBUFS) if drop is
requested, even if there is no CAN frame pending. As the i.MX25 is a
single core CPU, while the rx-offload processing is active, there is
no thread to process packets from the offload queue. So the queue
doesn't get any shorter and this results is a tight loop.

Instead of always returning ERR_PTR(-ENOBUFS) if drop is requested,
return NULL if no CAN frame is pending.

Changes since v1: https://lore.kernel.org/all/20220810144536.389237-1-u.kleine-koenig@pengutronix.de
- don't break in can_rx_offload_irq_offload_fifo() in case of an error,
  return NULL in flexcan_mailbox_read() in case of no pending CAN frame
  instead

Fixes: 4e9c9484 ("can: rx-offload: Prepare for CAN FD support")
Link: https://lore.kernel.org/all/20220811094254.1864367-1-mkl@pengutronix.de


Cc: stable@vger.kernel.org # v5.5
Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Thorsten Scherer <t.scherer@eckelmann.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>