unlock: Don't pass options again to overlayfs

There seems to be a tricky regression here with the util-linux
support for the new mount API, plus overlays support for it.

```
[2023-11-09T21:05:30.633Z] Nov 09 21:05:26 qemu0 kola-runext-unlock-transient.sh[2108]: + unshare -m -- /bin/sh -c 'mount -o remount,rw /usr && echo hello from transient unlock >/usr/share/writable-usr-test'
[2023-11-09T21:05:30.633Z] Nov 09 21:05:26 qemu0 kola-runext-unlock-transient.sh[2148]: mount: /usr: mount point not mounted or bad option.
[2023-11-09T21:05:30.633Z] Nov 09 21:05:26 qemu0 kola-runext-unlock-transient.sh[2148]:        dmesg(1) may have more information after failed mount system call.
```

OK this seems related to the new mount API support in util-linux and overlayfs.  From a strace:

```
2095  open_tree(AT_FDCWD, "/usr", OPEN_TREE_CLOEXEC) = 3
2095  mount_setattr(-1, NULL, 0, NULL, 0) = -1 EINVAL (Invalid argument)
...
2095  fspick(3, "", FSPICK_NO_AUTOMOUNT|FSPICK_EMPTY_PATH) = 4
2095  fsconfig(4, FSCONFIG_SET_FLAG, "seclabel", NULL, 0) = 0
2095  fsconfig(4, FSCONFIG_SET_STRING, "lowerdir", "usr", 0) = -1 EINVAL (Invalid argument)
```

I think the core problem here is it's trying to reconfigure the mount with existing options,
but in the new mount namespace we can't see the lowerdir.

Here we really really just want to remount writable.  Telling
util-linux to not pass existing options fixes it.

tests/kolainst/destructive/unlock-transient.sh

@@ -16,7 +16,7 @@ case "${AUTOPKGTEST_REBOOT_MARK:-}" in
       fatal "modified /usr"
     fi
     # But, we can affect it in a new mount namespace
-    unshare -m -- /bin/sh -c 'mount -o remount,rw /usr && echo hello from transient unlock >'"${testfile}"
+    unshare -m -- /bin/sh -c 'mount --options-source=disable -o remount,rw /usr && echo hello from transient unlock >'"${testfile}"
     assert_file_has_content "${testfile}" "hello from transient unlock"
     # Still can't write to it from the outer namespace
     if touch ${testfile} || rm -v "${testfile}" 2>/dev/null; then