staging: gasket: page_table: handle failed dma_map_page

Handle dma_map_page failing in gasket_alloc_extended_subtable: free
memory, don't add invalid page table entry.

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>