futex: split out futex value validation code

Refactor the code to validate the expected futex value in order to
reuse it with the requeue_pi code.

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