ovl: don't allow datadir only

In the Linux kernel, the following vulnerability has been resolved:

ovl: don't allow datadir only

In theory overlayfs could support upper layer directly referring to a data

layer, but there's no current use case for this.

Originally, when data-only layers were introduced, this wasn't allowed,

only introduced by the "datadir+" feature, but without actually handling

this case, resulting in an Oops.

Fix by disallowing datadir without lowerdir.