orangefs: fix timestamp updates from truncate

We need to update c/mtime in truncate and ftruncate, except for a
special case where we can skip them due to a lacking size change.

Also get rid of the lazy mtime flag - the iattr already has the
timestamp which we should use to set exactly that time.

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