orangefs: support very large directories

This works by maintaining a linked list of pages which the directory
has been read into rather than one giant fixed-size buffer.

This replaces code which limits the total directory size to the total
amount that could be returned in one server request.  Since filenames
are usually considerably shorter than the maximum, the old code could
usually handle several server requests before running out of space.

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