A set of PI futex fixes:

      - Address a longstanding issue where the user space part of the PI futex
     is not writeable. The kernel returns with inconsistent state which can
     in the worst case result in a UAF of a tasks kernel stack. The solution
     is to establish consistent kernel state which makes future operations
     on the futex fail because user space and kernel space state are
     inconsistent. Not a problem as PI futexes fundamentaly require a
     functional RW mapping and if user space pulls the rug under it, then it
     can keep the pieces it asked for.
 
   - Address an issue where the return value is incorrect in case that the
     futex was acquired after a timeout/signal made the waiter drop out of
     the rtmutex wait. In one of the corner cases the kernel returned an
     error code despite having successfully acquired the futex.
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