futex: rename field in futex_q to clarify single waiter semantics

Impact: simplify code

I've tripped over the naming of this field a couple times.

The futex_q uses a "waiters" list to represent a single blocked task and
then calles wake_up_all().

This can lead to confusion in trying to understand the intent of the code,
which is to have a single futex_q for every task waiting on a futex.

This patch corrects the problem, using a single pointer to the waiting
task, and an appropriate call to wake_up, rather than wake_up_all.

Compile and boot tested on an 8way x86_64 machine.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>