Processes can be restored in another mntns, so mnt_id will be
useless in this case. If mntns isn't dumped, we have to dump a path
to a mount point.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>