mkosi: workaround issue in mdadm-4.5 + linux-6.18

mkosi: workaround issue in mdadm-4.5 + linux-6.18

After debian updated the kernel from 6.17 to 6.18, mdadm command fails
with the following:
```
+ mdadm --create /dev/md/mdmirror ...
mdadm: size set to 64512K
mdadm: Can't open /sys/module/md_mod/parameters/legacy_async_del_gendisk
mdadm: init md module parameters fail
```

This seems a bug in mdadm, and fix is already merged in the upstream:
https://github.com/md-raid-utilities/mdadm/pull/228
Until the fix is backported, let's workaround the issue.

See also: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1125390