staging: gasket: page table: fixup error path allocating coherent mem

Correctly clean up data structure state in gasket_alloc_coherent_memory
error path, to ensure no double free on the stale pointer value.

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>