mm/vmscan: fix error return in kswapd_run()

Fix the error return value in kswapd_run().  The bug was introduced by
commit d5dc0ad928 ("mm/vmscan: fix error number for failed kthread").

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -3188,9 +3188,9 @@ int kswapd_run(int nid)
 	if (IS_ERR(pgdat->kswapd)) {
 		/* failure at boot is fatal */
 		BUG_ON(system_state == SYSTEM_BOOTING);
-		pgdat->kswapd = NULL;
 		pr_err("Failed to start kswapd on node %d\n", nid);
 		ret = PTR_ERR(pgdat->kswapd);
+		pgdat->kswapd = NULL;
 	}
 	return ret;
 }