ext4: switch to using write_cache_pages() for data=journal writeout

Instead of using generic_writepages(), let's use write_cache_pages() for
writeout of journalled data. It will allow us to stop providing
.writepage callback. Our data=journal writeback path would benefit from
a larger cleanup and refactoring but that's for a separate cleanup
series.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20221207112722.22220-10-jack@suse.cz


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