ext4: warn when dirtying page w/o buffers in data=journal mode

Recently I've got a report of BUG_ON trigerring during transaction
commit in ext4_journalled_writepage_callback() because we've spotted a
dirty page without buffers. Add WARN_ON_ONCE to
ext4_journalled_set_page_dirty() to catch the problematic condition
earlier where we have better chance of understanding which code path is
creating dirty data without preparing the page properly. Also update the
comment with current information when we are at it.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220310101832.5645-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>