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

pcistub_reg_add() is never called in atomic context.

pcistub_reg_add() is only called by pcistub_quirk_add, which is
only set in DRIVER_ATTR().

Despite never getting called from atomic context,
pcistub_reg_add() 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.

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

@@ -1149,7 +1149,7 @@ static int pcistub_reg_add(int domain, int bus, int slot, int func,
 	}
 	dev = psdev->dev;
 
-	field = kzalloc(sizeof(*field), GFP_ATOMIC);
+	field = kzalloc(sizeof(*field), GFP_KERNEL);
 	if (!field) {
 		err = -ENOMEM;
 		goto out;