Simplify qsbr online condition

Simplify qsbr online condition

Alan Stern wrote:

>       return v && (v - rcu_gp_ctr > ULONG_MAX / 2);
>
> Wouldn't it be better to write:
>
>       return v != 0 && v != rcu_gp_ctr;
>
> or something equivalent?  The point is, it's foolish to return 0 when
> rcu_gp_ctr has wrapped around to below v.  This can happen only when
> rcu_thread_online() is delayed (as by preemption), in which case we
> know that there's a good chance the algorithm will fail.  My version of
> the test eliminates the possibility of this failure, although it
> doesn't eliminate the possibility that rcu_gp_ctr has wrapped around so
> far that it is equal to v.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>