layers: do not try to unmount if not mounted at all on delete
layers: do not try to unmount if not mounted at all on delete

This check has been wrongly removed with #198
The check must stay as it's now part of the Stop/Delete API so reintroduce it back
This fixes #233 and the associated CRI-O issues
This PR + kubernetes-sigs/cri-o#1910 fully fix the issue
I'm going to revendor c/storage in CRI-O to full fix crio after this is merged

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@linux.com>

containers/storage,rhatdan/storage,rhatdan/storage,rhatdan/storage,containers/storage,containers/storage