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

pcistub_probe() is never called in atomic context.
This function is only set as ".probe" in struct pci_driver.

Despite never getting called from atomic context,
pcistub_probe() calls kmalloc() 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.

This is found by a static analysis tool named DCNS written by myself.
And I also manually check it.

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

@@ -577,7 +577,7 @@ static int pcistub_probe(struct pci_dev *dev, const struct pci_device_id *id)
 		}
 
 		if (!match) {
-			pci_dev_id = kmalloc(sizeof(*pci_dev_id), GFP_ATOMIC);
+			pci_dev_id = kmalloc(sizeof(*pci_dev_id), GFP_KERNEL);
 			if (!pci_dev_id) {
 				err = -ENOMEM;
 				goto out;