ovl: relax redirect/metacopy requirements for lower -> data redirect

Allow the special case of a redirect from a lower layer to a data layer
without having to turn on metacopy.  This makes the feature work with
userxattr, which in turn allows data layers to be usable in user
namespaces.

Minimize the risk by only enabling redirect from a single lower layer to a
data layer iff a data layer is specified.  The only way to access a data
layer is to enable this, so there's really no reason not to enable this.

This can be used safely if the lower layer is read-only and the
user.overlay.redirect xattr cannot be modified.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>