units: make sure initrd-cleanup.service terminates before switching to rootfs

units: make sure initrd-cleanup.service terminates before switching to rootfs

A follow-up for commit a8cb1dc3e0fa81aff.

Commit a8cb1dc3e0fa81aff made sure that initrd-cleanup.service won't be stopped
when initrd-switch-root.target is isolated.

However even with this change, it might happen that initrd-cleanup.service
survives the switch to rootfs (since it has no ordering constraints against
initrd-switch-root.target) and is stopped right after when default.target is
isolated. This led to initrd-cleanup.service entering in failed state as it
happens when oneshot services are stopped.

This patch along with a8cb1dc3e0fa81aff should fix issue #4343.

Fixes: #4343