get the pid namespace by following task_active_pid_ns()

get the pid namespace by following task_active_pid_ns()

When unsharing of pid namespace, though nsproxy->pid_ns_for_children is
new, the process is still in the orignal pid namespace, only the forked
children processes will be in the new pid namespace.
So it's not correct to get process's pid namespace by nsproxy->pid_ns_for_children,
should get pid namespace by task_active_pid_ns() way, i.e.
pid->numbers[pid->level].ns

Signed-off-by: Li Chengyuan chengyuanli@hotmail.com