fileio.c: fix incorrect mtime

fileio.c: fix incorrect mtime

A regression was introduced that caused the mtime of /etc/.updated
and /var/.updated to be the current time when systemd-update-done
ran instead of being copied from /usr.

This was nearly fixed, but due to fflush being called after mtime
was carefully set, it was overwritten with the current time.

Regression introduced in 872c40396384f9fa89b01baf4e739d28ed590299

A fix was just missed in 39c38d773fbe2b4f76ff29ecd3344640efb9a86c

Fixes #8806

(cherry picked from commit be83711c7eb6a09e0d0c529c427cebcae87c0c1f)