mm/vmscan.c: fix -Wunused-but-set-variable warning

We fix the following warning when building kernel with W=1:

  mm/vmscan.c:1362:6: warning: variable 'err' set but not used [-Wunused-but-set-variable]

Link: https://lkml.kernel.org/r/20210924181218.21165-1-songkai01@inspur.com
Signed-off-by: Kai Song <songkai01@inspur.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1337,7 +1337,6 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
 {
 	int target_nid = next_demotion_node(pgdat->node_id);
 	unsigned int nr_succeeded;
-	int err;
 
 	if (list_empty(demote_pages))
 		return 0;
@@ -1346,7 +1345,7 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
 		return 0;
 
 	/* Demotion ignores all cpuset and mempolicy settings */
-	err = migrate_pages(demote_pages, alloc_demote_page, NULL,
+	migrate_pages(demote_pages, alloc_demote_page, NULL,
 			    target_nid, MIGRATE_ASYNC, MR_DEMOTION,
 			    &nr_succeeded);