staging: gasket: cleanup if dma_map_page fails in gasket_perform_mapping

Previously pages would have never been unmapped in this case.

Signed-off-by: Nick Ewalt <nicholasewalt@google.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>