gets reset properly when the old parent exits before the child. A flag
is set in old parent process when the child is reparented in ptrace(2).
If it's set when process is exiting, all running processes have their
'old parent process' pointer checked and reset if appropriate. Also
change to use 'struct proc *' pointer directly, rather than pid_t.
This fixes security/14444 by David Sainty.
Reviewed by Christos Zoulas.
