lazy-pages: stop checking for zero pagemaps

A page that explicitly mapped to zero pfn or a page that is not present
should be treated in the same way, therefore the zero pagemaps are not
required and will be removed by the following commits.

travis-ci: success for revert zero pagemaps
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>