staging: gasket: page_table: use total_entries for max ext lvl0 page idx

The maximum number of entries in the page table is configurable at
initialization time and should be used in gasket_extended_lvl0_page_idx.

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>