ovl: make redirect/metacopy rejection consistent

When overlayfs finds a file with metacopy and/or redirect attributes and
the metacopy and/or redirect features are not enabled, then it refuses to
act on those attributes while also issuing a warning.

There was an inconsistency in not checking metacopy found from the index.

And also only warning on an upper metacopy if it found the next file on the
lower layer, while always warning for metacopy found on a lower layer.

Fix these inconsistencies and make the logic more straightforward, paving
the way for following patches to change when data redirects are allowed.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>