ext4: Convert ext4_bio_write_page() to use a folio

ext4: Convert ext4_bio_write_page() to use a folio

Remove several calls to compound_head() and the last caller of
set_page_writeback_keepwrite(), so remove the wrapper too.

Also export bio_add_folio() as this is the first caller from a module.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20230324180129.1220691-4-willy@infradead.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>