ext4: Turn mpage_process_page() into mpage_process_folio()

The page/folio is only used to extract the buffers, so this is a
simple change.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20230324180129.1220691-6-willy@infradead.org


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