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.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

arch/um/drivers/mconsole_kern.c

@@ -123,7 +123,7 @@ void mconsole_log(struct mc_request *req)
 
 void mconsole_proc(struct mc_request *req)
 {
-	struct vfsmount *mnt = current->nsproxy->pid_ns->proc_mnt;
+	struct vfsmount *mnt = task_active_pid_ns(current)->proc_mnt;
 	char *buf;
 	int len;
 	struct file *file;