ext4: convert bd_buddy_page to bd_buddy_folio

There is no need to make this a multi-page folio, so leave all the
infrastructure around it in pages.  But since we're locking it, playing
with its refcount and checking whether it's uptodate, it needs to move
to the folio API.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20240416172900.244637-3-willy@infradead.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>