Fix RCU_GP_CTR_BIT

Fix RCU_GP_CTR_BIT

> > You lost me on this one:
> >
> >     sizeof(long) << 2 = 0x10
> >
> > I could believe the following (run on a 32-bit machine):
> >
> >     1 << (sizeof(long) * 8 - 1) = 0x80000000
> >
> > Or, if you were wanting to use a bit halfway up the word, perhaps this:
> >
> >     1 << (sizeof(long) * 4 - 1) = 0x8000
> >
> > Or am I confused?
>
> Well, I am at least partly confused.  You were wanting a low-order bit,
> so you want to lose the "- 1" above.  Here are some of the possibilities:
>
>       sizeof(long) = 0x4
>       sizeof(long) << 2 = 0x10
>       1 << (sizeof(long) * 8 - 1) = 0x80000000
>       1 << (sizeof(long) * 4) = 0x10000
>       1 << (sizeof(long) * 4 - 1) = 0x8000
>       1 << (sizeof(long) * 2) = 0x100
>       1 << (sizeof(long) * 2 - 1) = 0x80
>
> My guess is that 1 << (sizeof(long) * 4) and 1 << (sizeof(long) * 2)
> are of the most interest.
>

Exactly. I'll change it to :

I somehow thought this define was used as a bit number rather than the
bit mask.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>