What this PR does / why we need it:

What
Changes getWritableType() to look at the parent snapshot's filesystem instead of the child's, which doesn't even exist at this point.

Why
When creating sandboxes/containers from overlaybd pause images, the first creation works correctly but subsequent attempts fall back to overlayfs mode instead of using overlaybd.

The getWritableType() function was checking the child snapshot's labels and filesystem for overlaybd metadata, but the child snapshot doesn't have a label, and the child snapshot directories don't exist yet at this point. Note that in createMountPoint we were handling this properly for the stype, but not the writeType.

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?