Bug#1086976: debcow refresh

Hi,

I've seen on the debian-devel mailing list that the optimization of
removing the fsync with cow raised some concerns[1].
Due to the nature of cow the performances doesn't change at all with
or without fsync(), so I've refreshed the patches keeping the sync as
before.
This avoids the ambiguity where the performance benefit comes from not
syncing, which is not true.

Regards,

[1] https://lists.debian.org/debian-devel/2024/12/msg00602.html