buffer: convert block_truncate_page() to use a folio

Support large folios in block_truncate_page() and avoid three hidden calls
to compound_head().

Link: https://lkml.kernel.org/r/20230612210141.730128-15-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Bob Peterson <rpeterso@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>