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