um: Ensure that a stub page cannot get unmapped
um: Ensure that a stub page cannot get unmapped

Trinity discovered an execution path such that a task
can unmap his stub page.

Reported-by: Toralf Förster <24e9a7e86225fc01f236a81fc86e902a74542aa1@gmx.de>
Signed-off-by: Richard Weinberger <320bca71fc381a4a025636043ca86e734e31cf8b@nod.at>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs