kdat: Don't open /run/criu.kdat if doesn't exists

When CRIU is called for a first time and the /run/criu.kdat file does
not exists, the following warning is shown:
        Warn  (criu/kerndat.c:847): Can't load /run/criu.kdat

This patch is replacing this warning with a more appropriate debug message.
        File /run/criu.kdat does not exist

Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>