netns: restore ops before calling ops_exit_list

ops has been iterated to first element when call pre_exit, and
it needs to restore from save_ops, not save ops to save_ops

Fixes: d7d99872 ("netns: add pre_exit method to struct pernet_operations")
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>