inventory: save dump_uptime for criu dump if track_mem is set

A set of images from criu dump can be used as a previous point, when we
are doing snapshots. In this case, each point contains a full set of
images.

https://github.com/checkpoint-restore/criu/issues/479



v2: return -1 if invertory_save_uptime failed

Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>