userns: Allow unprivileged reboot

In a container with its own pid namespace and user namespace, rebooting
the system won't reboot the host, but terminate all the processes in
it and thus have the container shutdown, so it's safe.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>