orangefs: pass slot index back to readpage.

When userspace deposits more than a page of data into the shared buffer,
we'll need to know which slot it is in when we get back to readpage
so that we can try to use the extra data to fill some extra pages.

Signed-off-by: Mike Marshall <hubcap@omnibond.com>
Signed-off-by: Martin Brandenburg <martin@omnibond.com>