ext4: use writeback_iter in ext4_journalled_submit_inode_data_buffers

Use writeback_iter directly instead of write_cache_pages for a nicer
code structure and less indirect calls.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250505091604.3449879-1-hch@lst.de


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