freezer: don't catch kernel threads

OpenVZ creates per-container kernel threads and they live in
container's cgroups. CRIU should skip these threads, it's impossiable to
get any state of them.

Signed-off-by: Andrew Vagin <avagin@virtuozzo.org>
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>