memfd: Use CONFIG_HAS_MEMFD to not fail on old kernels

memfd is available since 3.16 kernel so don't fail
building criu on earlier kernels but simply don't
use it on older kernels.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>