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

mkosi/mkosi.extra/usr/lib/modules-load.d/mdadm-workaround.conf

@@ -0,0 +1,10 @@
+# This is a workaround for the issue observed with mdadm-4.5 + linux-6.18.
+# ===
+# + 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
+# ===
+# See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1125390
+# Fixed by https://github.com/md-raid-utilities/mdadm/pull/228
+md_mod

src/shared/discover-image.c

@@ -2169,7 +2169,7 @@ int image_read_metadata(Image *i, const char *root, const ImagePolicy *image_pol
                                 /* userns_fd= */ -EBADF,
                                 flags);
                 if (r < 0)
-                        return log_debug_errno(r, "Failed to acquire medata from image '%s': %m", i->path);
+                        return log_debug_errno(r, "Failed to acquire metadata from image '%s': %m", i->path);
 
                 free_and_replace(i->hostname, m->hostname);
                 i->machine_id = m->machine_id;