xen: xenbus: Use helper function IS_ERR_OR_NULL()

Use IS_ERR_OR_NULL() to detect an error pointer or a null pointer
open-coding to simplify the code.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230817014736.3094289-1-lizetao1@huawei.com
Signed-off-by: Juergen Gross <jgross@suse.com>

Author: Li Zetao

drivers/xen/xenbus/xenbus_probe_frontend.c

@@ -429,7 +429,7 @@ static void xenbus_check_frontend(char *class, char *dev)
 		printk(KERN_DEBUG "XENBUS: frontend %s %s\n",
 				frontend, xenbus_strstate(fe_state));
 		backend = xenbus_read(XBT_NIL, frontend, "backend", NULL);
-		if (!backend || IS_ERR(backend))
+		if (IS_ERR_OR_NULL(backend))
 			goto out;
 		err = xenbus_scanf(XBT_NIL, backend, "state", "%i", &be_state);
 		if (err == 1)