ext4: Convert mpage_map_and_submit_buffers() to use filemap_get_folios()

The called functions all use pages, so just convert back to a page.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>