ext4: stop providing .writepage hook

ext4: stop providing .writepage hook

Now we don't need .writepage hook for anything anymore. Reclaim is
fine with relying on .writepages to clean pages and we often couldn't
do much from the .writepage callback anyway. We only need to provide
.migrate_folio callback for the ext4_journalled_aops - let's use
buffer_migrate_page_norefs() there so that buffers cannot be modified
under jdb2's hands as that can cause data corruption. For example when
commit code does writeout of transaction buffers in
jbd2_journal_write_metadata_buffer(), we don't hold page lock or have
page writeback bit set or have the buffer locked. So page migration
code would go and happily migrate the page elsewhere while the copy is
running thus corrupting data.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20221207112722.22220-12-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>