pidns: Use task_active_pid_ns where appropriate

The expressions tsk->nsproxy->pid_ns and task_active_pid_ns
aka ns_of_pid(task_pid(tsk)) should have the same number of
cache line misses with the practical difference that
ns_of_pid(task_pid(tsk)) is released later in a processes life.

Furthermore by using task_active_pid_ns it becomes trivial
to write an unshare implementation for the the pid namespace.

So I have used task_active_pid_ns everywhere I can.

In fork since the pid has not yet been attached to the
process I use ns_of_pid, to achieve the same effect.

[Note from Zheng Liu]
We need to backport this patch for fixing a panic in our cgroup system.
It will cause a panic when we stop a instance.

Reference: 17cf22c33e1f1b5e435469c84e43872579497653
Patch-mainline: v3.7-rc3-9-g17cf22c
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>