ext4: move the calculation of wbc->nr_to_write to mpage_folio_done()

ext4: move the calculation of wbc->nr_to_write to mpage_folio_done()

mpage_folio_done() should be a more appropriate place than
mpage_submit_folio() for updating the wbc->nr_to_write after we have
submitted a fully mapped folio. Preparing to make mpage_submit_folio()
allows to submit partially mapped folio that is still under processing.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Link: https://patch.msgid.link/20250707140814.542883-3-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>