cache: parse the correct field for offset

commit 10697a05a2 introduced the issue.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/chunked/cache_linux.go

@@ -474,7 +474,7 @@ func (c *layersCache) findDigestInternal(digest string) (string, string, int64,
 		if digest != "" {
 			position := string(layer.metadata.vdata[off : off+len])
 			parts := strings.SplitN(position, "@", 2)
-			offFile, _ := strconv.ParseInt(parts[1], 10, 64)
+			offFile, _ := strconv.ParseInt(parts[0], 10, 64)
 			return layer.target, parts[1], offFile, nil
 		}
 	}