[PATCH] fix a page dirtying race in vmscan.c

[PATCH] fix a page dirtying race in vmscan.c

There's a small window in which another CPU could dirty the page after
we've cleaned it, and before we've moved it to mapping->dirty_pages().
The end result is a dirty page on mapping->locked_pages, which is
wrong.

So take mapping->page_lock before clearing the dirty bit.