orangefs: simplify iget

pvfs2_iget_common is never called with the keep_locked argument set to true,
so remove it and fold pvfs2_iget into pvfs2_iget_common and update all the
comments describing it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>