restore: close_old_fds after restoring namespaces

Restore of netns uses the file descriptor on the root netns. This
descriptor is opened before forking.
We can add one more service fd, but I think this approach is better.

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