mm/vmscan: fix error number for failed kthread

Fix the return value while failing to create the kswapd kernel thread.
Also, the error message is prioritized as KERN_ERR.

Signed-off-by: Gavin Shan <[e-mail]>
Signed-off-by: Wanpeng Li <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/vmscan.c

···
 	if (IS_ERR(pgdat->kswapd)) {
 		/* failure at boot is fatal */
 		BUG_ON(system_state == SYSTEM_BOOTING);
-		printk("Failed to start kswapd on node %d\n",nid);
 		pgdat->kswapd = NULL;
-		ret = -1;
+		pr_err("Failed to start kswapd on node %d\n", nid);
+		ret = PTR_ERR(pgdat->kswapd);
 	}
 	return ret;
 }