staging: gasket: fix gasket_free_coherent_memory metadata frees

Free gasket_coherent_page_entries metadata memory, update data
structures accordingly.

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>