Fix EZP-21588: error in copy with split dfs tables

http://jira.ez.no/browse/EZP-21588

The `eZDFSFileHandlerMySQLBackend::copyInner()` method was using the source file path to get the table for the destination file.

Tested with eZ Image Editor, with DFS + split tables enabled: editing a file just doesn't work without the patch.