cxl/mbox: Allow for IRQ_NONE case in the isr

For cases when the mailbox background operation is not complete,
do not "handle" the interrupt, as it was not from this device.
And furthermore there are no racy scenarios such as the hw being
out of sync with the driver and starting a new background op
behind its back.
Reported-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Fixes: ccadf131 (cxl/mbox: Add background cmd handling machinery)
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20230612181038.14421-2-dave@stgolabs.netSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/pci.c

@@ -118,8 +118,10 @@ static irqreturn_t cxl_pci_mbox_irq(int irq, void *id)
 	struct cxl_dev_id *dev_id = id;
 	struct cxl_dev_state *cxlds = dev_id->cxlds;
 
+	if (!cxl_mbox_background_complete(cxlds))
+		return IRQ_NONE;
+
 	/* short-circuit the wait in __cxl_pci_mbox_send_cmd() */
-	if (cxl_mbox_background_complete(cxlds))
 	rcuwait_wake_up(&cxlds->mbox_wait);
 
 	return IRQ_HANDLED;