orangefs: implement xattr cache

This uses the same timeout as the getattr cache.  This substantially
increases performance when writing files with smaller buffer sizes.

When writing, the size is (often) changed, which causes a call to
notify_change which calls security_inode_need_killpriv which needs a
getxattr.  Caching it reduces traffic to the server.

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