orangefs: rewrite readdir to fix several bugs

In the past, readdir assumed that the user buffer will be large enough
that all entries from the server will fit.  If this was not true,
entries would be skipped.

Since it works now, request 512 entries rather than 96 per server
operation.

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