staging: gasket: page_table: don't unmap coherent pages

Only call dma_unmap_page if there was an associated dma_map_page call.

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>