udf: remove ->writepage

->writepage is a very inefficient method to write back data, and only
used through write_cache_pages or as a fallback when no ->migrate_folio
method is present.

Set ->migrate_folio to the generic buffer_head based helper, and remove
the ->writepage implementation in extfat.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>