VE: let ->ve_netns live a bit more
1. netns shutdown is done asynchronously
2. nsproxy free is done synchronously
which means we can't use "get_exec_env()->ve_ns->net_ns" construct
anywhere in netns teardown codepath. ->ve_ns will be NULL (fixable) or
will point to freed memory (hardly fixable).
The solution it to pin netns one more time, and use get_exec_env()->ve_netns .
get_exec_env() is always valid. It's ->ve_netns will also be valid during
shutdown. As for ->ve_ns, we don't care from now.