Fix composefs and kdump interaction

As part of https://github.com/coreos/fedora-coreos-tracker/issues/1718 and #11 (comment 1929130136), we are looking at unify on composefs.

We are having a weird issue with kdump when composefs is enabled:

• https://bugzilla.redhat.com/show_bug.cgi?id=2284097
• https://issues.redhat.com/browse/RHEL-35885

Is this tested with the bootc images?