This allows us to differentiate between filters and nodes with COW
backing files: Filters cannot be used as overlays at all (for this
function).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>