This part of code is responsible for reseting pagemap to proper locatation,
and mapping requested address to zero pfn if needed. The upcoming addtions
to uffd.c will reuse this code.

travis-ci: success for uffd: A new set of improvements
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>