Decoding a merge dir, whose origin's parent is under a redirected
lower dir is not always possible. As a simple aproximation, we do
not encode lower dir file handles when overlay has multiple lower
layers and origin is below the topmost lower layer.

We should later relax this condition and copy up only the parent
that is under a redirected lower.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>