zero_page_pfn should be invalid if it isn't set to a real value.

zero is bad, because pagemap returns zero pfn-s for non-root users.

$ python test/zdtm.py run -t zdtm/static/env00 -k always -f h --user --norst
========================== Run zdtm/static/env00 in h ==========================
Start test
./env00 --pidfile=env00.pid --outfile=env00.out --envname=ENV_00_TEST
Run criu dump
Wait for zdtm/static/env00 to die for 0.100000
========================= Test zdtm/static/env00 PASS ==========================

$ ./crit/crit show test/dump/zdtm/static/env00/24/1/pagemap-24.img
{
    "magic": "PAGEMAP",
    "entries": [
        {
            "pages_id": 1
        },
        {
            "vaddr": "0x7ffcfd5ed000",
            "nr_pages": 2
        }
    ]
}
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>