Inline relevant parts of get_page inside uffd_handle_page and call
uffd_{copy,zero}_page after we've got the data.

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>