This unwinds part of a25ae2b. The way the code ended up both
self.tree and self.mountpoint ended up pointing to the exactly
same path and so we'd end up doing two `umount -R` operations
on the same path. This ended up being a duplicate unmount.
On Fedora 39 this yields an error like:
```
mount/ostree.deployment (org.osbuild.ostree.deployment): umount: /var/osbuild/store/stage/uuid-efaac9370d25455d9e8df6d847ecb5b3/data/tree: not mounted
mount/ostree.deployment (org.osbuild.ostree.deployment): Traceback (most recent call last):
mount/ostree.deployment (org.osbuild.ostree.deployment): File "/var/b/shared/code/github.com/osbuild/osbuild/mounts/org.osbuild.ostree.deployment", line 136, in <module>
mount/ostree.deployment (org.osbuild.ostree.deployment): main()
mount/ostree.deployment (org.osbuild.ostree.deployment): File "/var/b/shared/code/github.com/osbuild/osbuild/mounts/org.osbuild.ostree.deployment", line 132, in main
mount/ostree.deployment (org.osbuild.ostree.deployment): service.main()
mount/ostree.deployment (org.osbuild.ostree.deployment): File "/var/b/shared/code/github.com/osbuild/osbuild/osbuild/host.py", line 252, in main
mount/ostree.deployment (org.osbuild.ostree.deployment): self.stop()
mount/ostree.deployment (org.osbuild.ostree.deployment): File "/var/b/shared/code/github.com/osbuild/osbuild/osbuild/mounts.py", line 126, in stop
mount/ostree.deployment (org.osbuild.ostree.deployment): self.umount()
mount/ostree.deployment (org.osbuild.ostree.deployment): File "/var/b/shared/code/github.com/osbuild/osbuild/mounts/org.osbuild.ostree.deployment", line 125, in umount
mount/ostree.deployment (org.osbuild.ostree.deployment): subprocess.run(["umount", "-R", self.tree],
mount/ostree.deployment (org.osbuild.ostree.deployment): File "/usr/lib64/python3.12/subprocess.py", line 571, in run
mount/ostree.deployment (org.osbuild.ostree.deployment): raise CalledProcessError(retcode, process.args,
mount/ostree.deployment (org.osbuild.ostree.deployment): subprocess.CalledProcessError: Command '['umount', '-R', '/var/osbuild/store/stage/uuid-efaac9370d25455d9e8df6d847ecb5b3/data/tree']
' returned non-zero exit status 1.
⏱ Duration: 103s
```
I think this was necessary because of a bug in util-linux that
mean some of the accounting information got out of date when
doing a `mount --move` operation, which we use here. I think this
bug (or bugs) is now fixed [1][2] in util-linux v2.39 (in Fedora 39),
which is now causing the above pasted error on F39.
Let's just add code here that mentions the problem and workaround
it with a loop to keep unmounting (essentially what the umount -R
should have done to overmounted filesystems if the mountinfo/utab
was correct) and also mention when we should be able to drop this
workaround.
[1] a04149fbb7
[2] 8cf6c50757
0da68e9af5