futex: add helper to find the top prio waiter of a futex

Improve legibility by wrapping finding the top waiter in a function.
This will be used by the follow-on patches for enabling requeue pi.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>