ext4: convert ac_buddy_page to ac_buddy_folio

This just carries around the bd_buddy_folio so should also be a folio.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20240416172900.244637-6-willy@infradead.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>