Bug#745717: di-utils: fetch-url runs mountmedia without cleaning up; blocks partitioner preseeding

[Zhang Cheng <zhangcheng@linuxdeepin.com>]

> I personally modified fetch-url/file and fetch-url/floppy, added `umount /media || true` before every return path, then the installer works incorrectly.

Sorry, typo here, then the installer works *correctly*.

BTW, this bug may not be triggered that frequently. fetch-url is
invoked such early that most filesystem modules are not modprobed,
thus will just fail to mount.
I'm using a self-compiled linux kernel with the d-i initrd.gz, I
compiled all needed filesystem modules into the kernel, so come into
this problem.