powerpc/powernv: add NULL check after kzalloc

Fixes coccicheck warning:

./arch/powerpc/platforms/powernv/opal.c:813:1-5:
	alloc with no test, possible model on line 814

Add NULL check after kzalloc.
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200509020838.121660-1-chenzhou10@huawei.com

arch/powerpc/platforms/powernv/opal.c

@@ -811,6 +811,10 @@ static int opal_add_one_export(struct kobject *parent, const char *export_name,
 		goto out;
 
 	attr = kzalloc(sizeof(*attr), GFP_KERNEL);
+	if (!attr) {
+		rc = -ENOMEM;
+		goto out;
+	}
 	name = kstrdup(export_name, GFP_KERNEL);
 	if (!name) {
 		rc = -ENOMEM;