Maintain manifest link compatibility

Unfortunately, the 2.1 releease has written manfiest links into the wrong
directory. This doesn't affect new 2.1 deployments but fixing this to be 2.0
backwards compatible has broken 2.1.0 compatibility. To ensure we have
compatibility between 2.0, 2.1.0 and future releases, we now check one of
several locations to identify a manifest link.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/signaturestore.go

@@ -132,10 +132,10 @@ func (s *signatureStore) linkedBlobStore(ctx context.Context, revision digest.Di
 		repository: s.repository,
 		blobStore:  s.blobStore,
 		blobAccessController: &linkedBlobStatter{
-			blobStore:  s.blobStore,
-			repository: s.repository,
-			linkPath:   linkpath,
+			blobStore:   s.blobStore,
+			repository:  s.repository,
+			linkPathFns: []linkPathFunc{linkpath},
 		},
-		linkPath: linkpath,
+		linkPathFns: []linkPathFunc{linkpath},
 	}
 }