overlay: avoid bogus error unmounting base layers
overlay: avoid bogus error unmounting base layers

When an overlay layer has no parent, "mounting" it is just a no-op,
since we're using its "diff" directory directly, and we didn't bother to
create a "merged" directory for the layer.  Check for this case when we
go to unmount the layer's "merged" directory, to avoid a spurious "not
mounted" error.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

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