xen: xen-pciback: Replace GFP_ATOMIC with GFP_KERNEL in pcistub_device_alloc

pcistub_device_alloc() is never called in atomic context.

The call chain ending up at pcistub_device_alloc() is:
[1] pcistub_device_alloc() <- pcistub_seize() <- pcistub_probe()
pcistub_probe() is only set as ".probe" in struct pci_driver.
This function is not called in atomic context.

Despite never getting called from atomic context,
pcistub_device_alloc() calls kzalloc() with GFP_ATOMIC,
which does not sleep for allocation.
GFP_ATOMIC is not necessary and can be replaced with GFP_KERNEL,
which can sleep and improve the possibility of sucessful allocation.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xen-pciback/pci_stub.c

@@ -71,7 +71,7 @@ static struct pcistub_device *pcistub_device_alloc(struct pci_dev *dev)
 
 	dev_dbg(&dev->dev, "pcistub_device_alloc\n");
 
-	psdev = kzalloc(sizeof(*psdev), GFP_ATOMIC);
+	psdev = kzalloc(sizeof(*psdev), GFP_KERNEL);
 	if (!psdev)
 		return NULL;