orangefs: implement statx

Fortunately OrangeFS has had a getattr request mask for a long time.

The server basically has two difficulty levels for attributes.  Fetching
any attribute except size requires communicating with the metadata
server for that handle.  Since all the attributes are right there, it
makes sense to return them all.  Fetching the size requires
communicating with every I/O server (that the file is distributed
across).  Therefore if asked for anything except size, get everything
except size, and if asked for size, get everything.

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