locking/qspinlock: Fix spin_unlock_wait() some more

While this prior commit:

  54cf809b ("locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()")

... fixes spin_is_locked() and spin_unlock_wait() for the usage
in ipc/sem and netfilter, it does not in fact work right for the
usage in task_work and futex.

So while the 2 locks crossed problem:

	spin_lock(A)		spin_lock(B)
	if (!spin_is_locked(B)) spin_unlock_wait(A)
	  foo()			foo();

... works with the smp_mb() injected by both spin_is_locked() and
spin_unlock_wait(), this is not sufficient for:

	flag = 1;
	smp_mb();		spin_lock()
	spin_unlock_wait()	if (!flag)
				  // add to lockless list
	// iterate lockless list

... because in this scenario, the store from spin_lock() can be delayed
past the load of flag, uncrossing the variables and loosing the
guarantee.

This patch reworks spin_is_locked() and spin_unlock_wait() to work in
both cases by exploiting the observation that while the lock byte
store can be delayed, the contender must have ...