change reader_data for reader_registry

change reader_data for reader_registry

> > In Figure 9, "reader_data" shouldn't be used as both a variable name
> > and a structure tag.  Furthermore, the text needs to explain what it
> > is.
>
> I will defer to Mathieu on this one, though it would certainly prevent
> using this code in a C++ program.
>

Yes, I've been lazy on the naming here. How about :

struct reader_registry {
        pthread_t tid;
        long *urcu_active_readers;
};

static struct reader_registry *registry;

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