xen/pciback: don't call pcistub_device_put() under lock

pcistub_device_put() shouldn't be called under spinlock, as it can
sleep.

For this reason pcistub_device_get_pci_dev() needs to be modified:
instead of always calling pcistub_device_get() just do the call of
pcistub_device_get() only if it is really needed. This removes the
need to call pcistub_device_put().

Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/lkml/Y+JUIl64UDmdkboh@kadam/
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Link: https://lore.kernel.org/r/20230328084549.20695-1-jgross@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xen-pciback/pci_stub.c

@@ -194,8 +194,6 @@ static struct pci_dev *pcistub_device_get_pci_dev(struct xen_pcibk_device *pdev,
 	struct pci_dev *pci_dev = NULL;
 	unsigned long flags;
 
-	pcistub_device_get(psdev);
-
 	spin_lock_irqsave(&psdev->lock, flags);
 	if (!psdev->pdev) {
 		psdev->pdev = pdev;
@@ -203,8 +201,8 @@ static struct pci_dev *pcistub_device_get_pci_dev(struct xen_pcibk_device *pdev,
 	}
 	spin_unlock_irqrestore(&psdev->lock, flags);
 
-	if (!pci_dev)
-		pcistub_device_put(psdev);
+	if (pci_dev)
+		pcistub_device_get(psdev);
 
 	return pci_dev;
 }