mm: page->mapping folio->mapping should have the same offset

As with the other members of folio, the offset of page->mapping and
folio->mapping must be the same.  The compile-time check was
inadvertently removed during development.  Add it back.

[willy@infradead.org: changelog redo]

Link: https://lkml.kernel.org/r/20220104011734.21714-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: RichardWeiYang

include/linux/mm_types.h

@@ -261,6 +261,7 @@ static_assert(sizeof(struct page) == sizeof(struct folio));
 	static_assert(offsetof(struct page, pg) == offsetof(struct folio, fl))
 FOLIO_MATCH(flags, flags);
 FOLIO_MATCH(lru, lru);
+FOLIO_MATCH(mapping, mapping);
 FOLIO_MATCH(compound_head, lru);
 FOLIO_MATCH(index, index);
 FOLIO_MATCH(private, private);