mm, thp: clear PG_mlocked when last mapping gone

I missed clear_page_mlock() in page_remove_anon_compound_rmap().
It usually shouldn't cause any problems since we munlock pages
explicitly, but in conjunction with missed munlock in __oom_reap_vmas()
it causes problems:
 http://lkml.kernel.org/r/5661FBB6.6050307@oracle.com

Let's put it in place an mirror behaviour for small pages.

NOTE: I'm not entirely sure why we ever need clear_page_mlock() in
page_remove_rmap() codepath. It looks redundant to me as we munlock
pages anyway. But this is out of scope of the patch.

The patch can be folded into
 "thp: allow mlocked THP again"

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: kiryl

mm/rmap.c

@@ -1282,6 +1282,9 @@ static void page_remove_anon_compound_rmap(struct page *page)
 		nr = HPAGE_PMD_NR;
 	}
 
+	if (unlikely(PageMlocked(page)))
+		clear_page_mlock(page);
+
 	if (nr) {
 		__mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
 		deferred_split_huge_page(page);