posix-timers: Use a callback for cancel synchronization on PREEMPT_RT

Posix timer delete retry loops are affected by the same priority inversion
and live lock issues as the other timers.
    
Provide a RT specific synchronization function which keeps a reference to
the timer by holding rcu read lock to prevent the timer from being freed,
dropping the timer lock and invoking the timer specific wait function via a
new callback.
    
This does not yet cover posix CPU timers because they need more special
treatment on PREEMPT_RT.

[ This is folded into the original attempt which did not use a callback. ]

Originally-by: Anna-Maria Gleixenr <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Frederic Weisbecker <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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