pidns: use task_active_pid_ns in do_notify_parent

pidns: use task_active_pid_ns in do_notify_parent

Using task_active_pid_ns is more robust because it works even after we
have called exit_namespaces.  This change allows us to have parent
processes that are zombies.  Normally a zombie parent processes is crazy
and the last thing you would want to have but in the case of not letting
the init process of a pid namespace be reaped until all of it's children
are dead and reaped a zombie parent process is exactly what we want.

Signed-off-by: Eric W. Biederman <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Louis Rilling <[email protected]>
Cc: Mike Galbraith <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>