buffer: Add folio_buffers()

While there is no intent to use large folios in filesystems using buffer
heads, converting the filesystems to use single-page folios is still worth
doing to remove legacy infrastructure and hidden calls to compound_head().
These helper functions are needed for that conversion to take place.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Acked-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Tested-by: Mike Marshall <hubcap@omnibond.com> # orangefs
Tested-by: David Howells <dhowells@redhat.com> # afs

include/linux/buffer_head.h

@@ -144,6 +144,7 @@ BUFFER_FNS(Defer_Completion, defer_completion)
 		((struct buffer_head *)page_private(page));	\
 	})
 #define page_has_buffers(page)	PagePrivate(page)
+#define folio_buffers(folio)		folio_get_private(folio)
 
 void buffer_check_dirty_writeback(struct page *page,
 				     bool *dirty, bool *writeback);